diff -r 9a94239e4f12 -r 067add650129 mercurial/extensions.py
--- a/mercurial/extensions.py	Mon Mar 13 08:53:31 2017 -0700
+++ b/mercurial/extensions.py	Mon Mar 13 09:11:08 2017 -0700
@@ -109,7 +109,7 @@
     """Portably format an import error into a form suitable for
     %-formatting into bytestrings."""
     if pycompat.ispy3:
-        return encoding.tolocal(str(inst).encode('utf-8'))
+        return encoding.unitolocal(str(inst))
     return inst
 
 def _reportimporterror(ui, err, failed, next):