Index: Lib/argparse.py
===================================================================
--- Lib/argparse.py	(revision 86743)
+++ Lib/argparse.py	(working copy)
@@ -1125,10 +1125,14 @@
                 raise ValueError(msg)
 
         # all other arguments are used as file names
-        if self._bufsize:
-            return open(string, self._mode, self._bufsize)
-        else:
-            return open(string, self._mode)
+        try:
+            if self._bufsize:
+                return open(string, self._mode, self._bufsize)
+            else:
+                return open(string, self._mode)
+        except IOError:
+            err = _sys.exc_info()[1]
+            raise ValueError("IOError: %s" % err)
 
     def __repr__(self):
         args = [self._mode, self._bufsize]
@@ -2228,9 +2232,10 @@
 
         # TypeErrors or ValueErrors also indicate errors
         except (TypeError, ValueError):
+            err = _sys.exc_info()[1]
             name = getattr(action.type, '__name__', repr(action.type))
-            msg = _('invalid %s value: %r')
-            raise ArgumentError(action, msg % (name, arg_string))
+            msg = _('invalid %s value: %r details: "%s"')
+            raise ArgumentError(action, msg % (name, arg_string, err))
 
         # return the converted value
         return result