py3: use forcebytestr() to stringify hook exception

This fixes test-narrow-exchange.t.

mercurial/hook.py

@@ -102,7 +102,7 b' def pythonhook(ui, repo, htype, hname, f'
                          (hname, exc.args[0]))
         else:
             ui.warn(_('error: %s hook raised an exception: '
-                      '%s\n') % (hname, encoding.strtolocal(str(exc))))
+                      '%s\n') % (hname, stringutil.forcebytestr(exc)))
         if throw:
             raise
         if not ui.tracebackflag: