hgweb: propagate http headers from ErrorResponse for web interface commands

This makes it possible for e.g. authorization hooks to provide appropriate
headers to make the web browser ask for credentials.

It's done in the same way as the existing code in wireprotoserver.py.

mercurial/hgweb/hgweb_mod.py

@@ -441,6 +441,8 b' class hgweb(object):'
             res.headers['Content-Type'] = ctype
             return rctx.sendtemplate('error', error=pycompat.bytestr(e))
         except ErrorResponse as e:
+            for k, v in e.headers:
+                res.headers[k] = v
             res.status = statusmessage(e.code, pycompat.bytestr(e))
             res.headers['Content-Type'] = ctype
             return rctx.sendtemplate('error', error=pycompat.bytestr(e))