[controller] UndoController: fix output method (closes: #1776091) Fix buggy direct output in `UndoController.publish()`. Raise now a ValidationError instead of malformed string Changes: * raise ValidationError when an error occured in undo * use the new message mechanism based on _cwmsgid If errors are not fatal, you have to change publish() method.

#!/usr/bin/env python
from cubicweb.cwctl import run
import sys
run(sys.argv[1:])