Import ValidationError from django.core.exceptions instead of django.forms.
Review Request #4608 — Created Sept. 20, 2013 and submitted
Import ValidationError from django.core.exceptions instead of django.forms.
They haven't announced any official deprecation of this, but it seems like
exceptions are being consolidated in one place. This change also marks some of
the validation strings for translation which we had missed.
Ran unit tests
