py3: use util.forcebytestr() to convert error messages to bytes

This makes the python 3 buildbot green again.

Differential Revision: https://phab.mercurial-scm.org/D4811

mercurial/revlog.py

@@ -2669,7 +2669,8 b' class revlog(object):'
                     state['skipread'].add(node)
             except Exception as e:
                 yield revlogproblem(
-                    error=_('unpacking %s: %s') % (short(node), e),
+                    error=_('unpacking %s: %s') % (short(node),
+                                                   stringutil.forcebytestr(e)),
                     node=node)
                 state['skipread'].add(node)