hgweb: add a `message` attribute to `hgweb.common.ErrorResponse`

This fixes a Python 3 bug where hgweb assumes an Exception
subclass will have a `.message` attribute after running
`Exception.__init__`.[1] The Python 3 way to get this info would
be `e.args[0]`, but adding a new named attribute is more
ergonomic in my view.

[1] https://www.mercurial-scm.org/repo/hg-committed/file/6ccf539aec71/mercurial/hgweb/hgwebdir_mod.py#l459

Differential Revision: https://phab.mercurial-scm.org/D6840

mercurial/hgweb/common.py

@@ -106,6 +106,7 b' class ErrorResponse(Exception):'
         if headers is None:
             headers = []
         self.headers = headers
+        self.message = message
 
 class continuereader(object):
     """File object wrapper to handle HTTP 100-continue.