subrepo: use util.forcebytestr() instead of str() on exception

Differential Revision: https://phab.mercurial-scm.org/D2437

mercurial/subrepo.py

@@ -73,7 +73,8 b' def annotatesubrepoerror(func):'
             raise ex
         except error.Abort as ex:
             subrepo = subrelpath(self)
-            errormsg = str(ex) + ' ' + _('(in subrepository "%s")') % subrepo
+            errormsg = (util.forcebytestr(ex) + ' '
+                        + _('(in subrepository "%s")') % subrepo)
             # avoid handling this exception by raising a SubrepoAbort exception
             raise SubrepoAbort(errormsg, hint=ex.hint, subrepo=subrepo,
                                cause=sys.exc_info())