New validation model should invalidate on custom plugin validation

With the change to validation, we weren't able to invalidate a
page and changes by throwing an exception from plugin.on_change
because plugin.__invalid_changes didn't get populated by custom
validation. Do so.

Change-Id: Ib2b1943481af377ce629593725de03c9f6d58868
Signed-off-by: Ryan Barry <rbarry@redhat.com>