Fixed #22915 -- Document backward incompatible changes in the Validation...

...Error constructor.

This patch also fixes update_error_dict to better handle the use case described
in this ticket, previously the type of the provided container could be lost in
some conditions.
I would try to avoid the warning box. First, introduce the way to upgrade, then mention the way to preserve backwards compat, like the changed note in https://docs.djangoproject.com/en/1.7/topics/forms/formsets/#django.forms.formsets.BaseFormSet.can_delete