Rename add_error methods: Django 1.7 conflict

In Django 1.7, Forms have an add_error() method and our definition
was conflicting with Django's method. Thanks to hertzog@debian.org
for the patch.

Close-bug: #1352919
Change-Id: I1f558cf7f3198a68629738a011e9cae0c3749320
Merge "Rename add_error methods: Django 1.7 conflict"