Fixed #12698. Model.clean() used with a ModelForm no longer causes a KeyError...

Fixed #12698. Model.clean() used with a ModelForm no longer causes a KeyError when raising a ValidationError.
Note that previously it was possible to raise a ValidationError in the same place with a message_dict attribute. That behavior was a bug and will no longer have the same behavior.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12402 bcc190cf-cafb-0310-a4f2-bffc1f526a37