I can confirm that without the patch the filename attribute is None
despite being mentioned in strerror.

Travis, you should use `orig_executable` instead of `args[0]` to cover:

  subprocess.call("exit 0", shell=True, executable='/nonexistent bash')

case. 

And use `cwd` if `child_exec_never_called`, to be consistent with the error message (see if/else statements above the raise statement).

It seems appropriate to set filename even if errno is not ENOENT
but to be conservative, you could provide filename iff `err_msg` is also changed i.e., iff errno is ENOENT.