[migration] litle enhancements and a fix to patch accidentally pushed two revisions ago

[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.

[migration] closes #1777411: schema modification commands shouldn't crash if already exists