hgweb: in protocol adapter, avoid control reaching end of non-void function

This greatly confounded some Python 3 porting work, once it was
managing to get this far.

Differential Revision: https://phab.mercurial-scm.org/D962

mercurial/hgweb/protocol.py

@@ -15,6 +15,7 b' from .common import ('
 )
 
 from .. import (
+    error,
     util,
     wireproto,
 )
@@ -199,3 +200,4 b' def call(repo, req, cmd):'
         rsp = rsp.message
         req.respond(HTTP_OK, HGERRTYPE, body=rsp)
         return []
+    raise error.ProgrammingError('hgweb.protocol internal failure', rsp)