Switch to a custom NotImplemented error

Some code in the drivers might actually raise the standard
NotImplemented error, and that would be a bug to skip a test if that was
the case. In that case we want the test to fail.

So let's switch to a custom exception that is used to skip the test if
it's raised.

Change-Id: Ideafee0b1f008ff32724fb98d6a477bd3976104d