Verbose not implemented errors

Hello,

I'm working on code.djangoproject.com/ticket/20841 for the Django Sprints today. This is a small part of the work to be done, as NotImplementedErrors are plentiful and they almost all could use semi-custom messages.

I've been differing verbage to any existing comments, and am writing the same 'subclasses of XX must provide YY method', as seen below.

Let me know if this works.