diff -r 01c1ee4bd1dd -r 1b2b727a885f mercurial/extensions.py
--- a/mercurial/extensions.py	Wed Jul 18 07:51:20 2012 -0700
+++ b/mercurial/extensions.py	Fri Jul 06 18:41:25 2012 +0200
@@ -42,7 +42,12 @@
         fd, fpath, desc = imp.find_module(f, [d])
         return imp.load_module(module_name, fd, fpath, desc)
     else:
-        return imp.load_source(module_name, path)
+        try:
+            return imp.load_source(module_name, path)
+        except IOError, exc:
+            if not exc.filename:
+                exc.filename = path # python does not fill this
+            raise
 
 def load(ui, name, path):
     # unused ui argument kept for backwards compatibility