Make `django.forms.util.ValidationError` and `django.core.exceptions.ValidationError` share ancestry

[Oliver Beattie]

There appears to be two (similarly named) ValidationErrors in Django — one living in django.core, and a form-specific one living in django.forms.util. I can appreciate that they have slightly differing uses, but it does seem like they have a lot in common. So, there would be a few routes one could take here:-

1. Alias django.forms.util.ValidationError to the django.core one

2. Make the forms version a subclass of the django.core one

3. Make each share a common base exception

Given how the exceptions are used, I'd say #2 would seem to make the most sense here, and wouldn't break anything. It's already rather confusing having two similarly-named exceptions, so I couldn't really see it being any more confusing to do this.

[Honza Král]

This is fixed in the model-validation branch (#6845) where there is only one ValidationError used throughout Django.

[Honza Král]

With model-validation merged, this is fixed in [12098]