Make NonBraindamagedErrorMixin use error_class

Form users can override the error_class (this is useful for rendering)
on a form.
Use NON_FIELD_ERRORS constant instead of hardcoded value

In the future, django team can decide to change the key associated to
non fields errors. They'll do so by changing NON_FIELD_ERRORS constant.

With this fix, django team won't be able to break betterforms.
Fixed Changlist typo.
Merge remote-tracking branch 'origin/nonbraindamagederrormixin-cleanup'