FIX: raise NotImplementedError, not NotImplemented

or else raising this error raises an error:

TypeError: 'NotImplementedType' object is not callable
Comment: NotImplemented is returned, while NotImplementedError is raised
FIX: raise NotImplementedError, not NotImplemented
Merge pull request #2828 from endolith/patch-2

BUG: NotImplementedError != NotImplemented