Raise NotImplementedError instead of NotImplemented

NotImplementedError is the name of the exception
(https://docs.python.org/2/library/exceptions.html).

NotImplemented is the name of a constant
(https://docs.python.org/2/library/constants.html).

It makes no sense to raise a constant. The exception should
be raised instead.

Change-Id: I369b47dac60106f96aed9d044f8b510afd09d266
Closes-Bug: #1339855