hgweb: pass exception message to builtin Exception ctor as sysstr

If we don't do this, the bytes gets repr()ed on Python 3 and we get
bogus error strings sent to clients. Ick.

Differential Revision: https://phab.mercurial-scm.org/D2447

mercurial/hgweb/common.py

@@ -93,7 +93,7 b' class ErrorResponse(Exception):'
     def __init__(self, code, message=None, headers=None):
         if message is None:
             message = _statusmessage(code)
-        Exception.__init__(self, message)
+        Exception.__init__(self, pycompat.sysstr(message))
         self.code = code
         if headers is None:
             headers = []