py3: cast exception to bytes

In order to appease Python 3.

Differential Revision: https://phab.mercurial-scm.org/D4733

mercurial/wireprotoframing.py

@@ -827,9 +827,10 b' class serverreactor(object):'
                     break
 
                 except Exception as e:
-                    for frame in createerrorframe(stream, requestid,
-                                                  '%s' % e,
+                    for frame in createerrorframe(
+                        stream, requestid, '%s' % stringutil.forcebytestr(e),
                                                   errtype='server'):
+
                         yield frame
 
                     break