encoding: protect against non-ascii default encoding

If the default python encoding was changed from ascii, the attempt to
encode as ascii before lower() could throw a UnicodeEncodeError.
Catch UnicodeError instead to prevent an unhandled exception.

mercurial/encoding.py

@@ -169,7 +169,7 b' def lower(s):'
     "best-effort encoding-aware case-folding of local string s"
     try:
         return s.encode('ascii').lower()
-    except UnicodeDecodeError:
+    except UnicodeError:
         pass
     try:
         if isinstance(s, localstr):