py3: use pycompat.bytestr() to convert error instances to bytes

Differential Revision: https://phab.mercurial-scm.org/D2516

mercurial/commands.py

@@ -2370,7 +2370,7 b' def grep(ui, repo, pattern, *pats, **opt'
     try:
         regexp = util.re.compile(pattern, reflags)
     except re.error as inst:
-        ui.warn(_("grep: invalid match pattern: %s\n") % inst)
+        ui.warn(_("grep: invalid match pattern: %s\n") % pycompat.bytestr(inst))
         return 1
     sep, eol = ':', '\n'
     if opts.get('print0'):

mercurial/debugcommands.py

@@ -1591,7 +1591,8 b' def debugobsolete(ui, repo, precursor=No'
                                      metadata=metadata, ui=ui)
                 tr.close()
             except ValueError as exc:
-                raise error.Abort(_('bad obsmarker input: %s') % exc)
+                raise error.Abort(_('bad obsmarker input: %s') %
+                                  pycompat.bytestr(exc))
             finally:
                 tr.release()
         finally: