Avoid “error: None” messages from distutils (#4931).

Thanks to Amaury Forgeot d’Arc and Philip J. Eby.
diff --git a/Lib/distutils/dir_util.py b/Lib/distutils/dir_util.py
index 5026e24..e2dc6f4 100644
--- a/Lib/distutils/dir_util.py
+++ b/Lib/distutils/dir_util.py
@@ -185,7 +185,6 @@
     Any errors are ignored (apart from being reported to stdout if 'verbose'
     is true).
     """
-    from distutils.util import grok_environment_error
     global _path_created
 
     if verbose >= 1:
@@ -202,8 +201,7 @@
             if abspath in _path_created:
                 del _path_created[abspath]
         except (IOError, OSError), exc:
-            log.warn(grok_environment_error(
-                    exc, "error removing %s: " % directory))
+            log.warn("error removing %s: %s", directory, exc)
 
 def ensure_relative(path):
     """Take the full path 'path', and make it a relative path.