Output below, first without DISTUTILS_DEBUG (showing the not very useful default message) and second with. (The actual exception is "Access to the path 'C:\Program Files\IronPython 2.7\Lib\site-packages\mock.py' is denied.")

This is with IronPython 2.7A1 which includes the Python 2.7 standard library.

> ipy setup.py build
running build
running build_py
warning: build_py: byte-compiling is disabled, skipping.

C:\compile\mock
> ipy setup.py install
running install
running build
running build_py
warning: build_py: byte-compiling is disabled, skipping.

running install_lib
copying build\lib\mock.py -> C:\Program Files\IronPython 2.7\Lib\site-packages
error: C:\Program Files\IronPython 2.7\Lib\site-packages\mock.py: None
C:\compile\mock
> set DISTUTILS_DEBUG=1

C:\compile\mock
> ipy setup.py install
options (after parsing config files):
options (after parsing command line):
option dict for 'install' command:
  {}
option dict for 'sdist' command:
  {'force_manifest': ('setup.cfg', '1')}
running install
Distribution.get_command_obj(): creating 'install' command object
pre-finalize_{unix,other}:
  prefix: None
  exec_prefix: None
  home: None
  user: 0
  install_base: None
  install_platbase: None
  root: None
  install_purelib: None
  install_platlib: None
  install_lib: None
  install_headers: None
  install_scripts: None
  install_data: None
  compile: None
  compile: True
  optimize: None
  force: None
  skip_build: 0
  record: None
post-finalize_{unix,other}():
  prefix: C:\Program Files\IronPython 2.7
  exec_prefix: None
  home: None
  user: 0
  install_base: C:\Program Files\IronPython 2.7
  install_platbase: C:\Program Files\IronPython 2.7
  root: None
  install_purelib: $base/Lib/site-packages
  install_platlib: $base/Lib/site-packages
  install_lib: None
  install_headers: $base/Include/$dist_name
  install_scripts: $base/Scripts
  install_data: $base
  compile: None
  compile: True
  optimize: None
  force: None
  skip_build: 0
  record: None
post-expand_basedirs():
  prefix: C:\Program Files\IronPython 2.7
  exec_prefix: None
  home: None
  user: 0
  install_base: C:\Program Files\IronPython 2.7
  install_platbase: C:\Program Files\IronPython 2.7
  root: None
  install_purelib: $base/Lib/site-packages
  install_platlib: $base/Lib/site-packages
  install_lib: None
  install_headers: $base/Include/$dist_name
  install_scripts: $base/Scripts
  install_data: $base
  compile: None
  compile: True
  optimize: None
  force: None
  skip_build: 0
  record: None
config vars:
{'base': 'C:\\Program Files\\IronPython 2.7',
 'dist_fullname': 'mock-0.7.0',
 'dist_name': 'mock',
 'dist_version': '0.7.0',
 'exec_prefix': 'C:\\Program Files\\IronPython 2.7',
 'platbase': 'C:\\Program Files\\IronPython 2.7',
 'prefix': 'C:\\Program Files\\IronPython 2.7',
 'py_version': '2.7.0',
 'py_version_nodot': '27',
 'py_version_short': '2.7',
 'sys_exec_prefix': 'C:\\Program Files\\IronPython 2.7',
 'sys_prefix': 'C:\\Program Files\\IronPython 2.7',
 'userbase': 'C:\\Users\\michael\\AppData\\Roaming\\Python',
 'usersite': 'C:\\Users\\michael\\AppData\\Roaming\\Python\\Python27\\site-packa
ges'}
post-expand_dirs():
  prefix: C:\Program Files\IronPython 2.7
  exec_prefix: None
  home: None
  user: 0
  install_base: C:\Program Files\IronPython 2.7
  install_platbase: C:\Program Files\IronPython 2.7
  root: None
  install_purelib: C:\Program Files\IronPython Lib/site-packages
  install_platlib: C:\Program Files\IronPython Lib/site-packages
  install_lib: None
  install_headers: C:\Program Files\IronPython Include/mock
  install_scripts: C:\Program Files\IronPython 2.7/Scripts
  install_data: C:\Program Files\IronPython 2.7
  compile: None
  compile: True
  optimize: None
  force: None
  skip_build: 0
  record: None
after prepending root:
  prefix: C:\Program Files\IronPython 2.7
  exec_prefix: None
  home: None
  user: 0
  install_base: C:\Program Files\IronPython 2.7
  install_platbase: C:\Program Files\IronPython 2.7
  root: None
  install_purelib: C:\Program Files\IronPython 2.7\Lib\site-packages
  install_platlib: C:\Program Files\IronPython 2.7\Lib\site-packages
  install_lib: C:\Program Files\IronPython 2.7\Lib\site-packages\
  install_headers: C:\Program Files\IronPython 2.7\Include\mock
  install_scripts: C:\Program Files\IronPython 2.7\Scripts
  install_data: C:\Program Files\IronPython 2.7
  compile: None
  compile: True
  optimize: None
  force: None
  skip_build: 0
  record: None
Distribution.get_command_obj(): creating 'build' command object
running build
running build_py
Distribution.get_command_obj(): creating 'build_py' command object
warning: build_py: byte-compiling is disabled, skipping.

running install_lib
Distribution.get_command_obj(): creating 'install_lib' command object
copying build\lib\mock.py -> C:\Program Files\IronPython 2.7\Lib\site-packages
error: C:\Program Files\IronPython 2.7\Lib\site-packages\mock.py: None
Traceback (most recent call last):
  File "setup.py", line 69, in <module>
  File "C:\Program Files\IronPython 2.7\Lib\distutils\core.py", line 151, in set
up
  File "C:\Program Files\IronPython 2.7\Lib\distutils\dist.py", line 952, in run
_commands
  File "C:\Program Files\IronPython 2.7\Lib\distutils\dist.py", line 971, in run
_command
  File "C:\Program Files\IronPython 2.7\Lib\distutils\command\install.py", line
575, in run
  File "C:\Program Files\IronPython 2.7\Lib\distutils\cmd.py", line 326, in run_
command
  File "C:\Program Files\IronPython 2.7\Lib\distutils\dist.py", line 971, in run
_command
  File "C:\Program Files\IronPython 2.7\Lib\distutils\command\install_lib.py", l
ine 97, in run
  File "C:\Program Files\IronPython 2.7\Lib\distutils\command\install_lib.py", l
ine 115, in install
  File "C:\Program Files\IronPython 2.7\Lib\distutils\cmd.py", line 373, in copy
_tree
  File "C:\Program Files\IronPython 2.7\Lib\distutils\dir_util.py", line 159, in
 copy_tree
  File "C:\Program Files\IronPython 2.7\Lib\distutils\file_util.py", line 148, i
n copy_file
  File "C:\Program Files\IronPython 2.7\Lib\distutils\file_util.py", line 44, in
 _copy_file_contents
IOError: System.IO.IOException: Access to the path 'C:\Program Files\IronPython
2.7\Lib\site-packages\mock.py' is denied. ---> System.UnauthorizedAccessExceptio
n: Access to the path 'C:\Program Files\IronPython 2.7\Lib\site-packages\mock.py
' is denied.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, I
nt32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions o
ptions, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolea
n useLongPath)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access,
FileShare share)
   at Microsoft.Scripting.PlatformAdaptationLayer.OpenInputFileStream(String pat
h, FileMode mode, FileAccess access, FileShare share)
   at IronPython.Runtime.PythonFile.__init__(CodeContext context, String name, S
tring mode, Int32 buffering)
   --- End of inner exception stack trace ---
   at Microsoft.Scripting.Runtime.LightExceptions.CheckAndThrow(Object value)
   at Microsoft.Scripting.Interpreter.FuncCallInstruction`2.Run(InterpretedFrame
 frame)
   at Microsoft.Scripting.Interpreter.Interpreter.HandleException(InterpretedFra
me frame, Exception exception)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.LightLambda.Run2[T0,T1,TRet](T0 arg0, T1 a
rg1)
   at IronPython.Compiler.PythonCallTargets.OriginalCallTarget1(PythonFunction f
unction, Object arg0)
   at Microsoft.Scripting.Interpreter.FuncCallInstruction`4.Run(InterpretedFrame
 frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.LightLambda.Run4[T0,T1,T2,T3,TRet](T0 arg0
, T1 arg1, T2 arg2, T3 arg3)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute3[T0,T1,T2,TRet](CallSite s
ite, T0 arg0, T1 arg1, T2 arg2)
   at Microsoft.Scripting.Interpreter.DynamicInstruction`4.Run(InterpretedFrame
frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.LightLambda.Run1[T0,TRet](T0 arg0)
   at IronPython.Compiler.RuntimeScriptCode.InvokeTarget(Scope scope)
   at IronPython.Compiler.RuntimeScriptCode.Run(Scope scope)
   at IronPython.Hosting.PythonCommandLine.RunFileWorker(String fileName)
   at IronPython.Hosting.PythonCommandLine.RunFile(String fileName)
C:\compile\mock
>