Raise more description exception for verify_policy_scoped

A friend recently came to me asking why his app was raising an error claiming that authorization was not performed when `authorize` was clearly being called. It took me a moment to guess that the same exception type was being used for both `verify_*` methods.

This struct me as counterintuitive and easily remedied by a more description error type for this case. My only concern was backward compatibility, which I opted to solve by making the new type a sub-type of the existing one. While this isn't a solution I love, it seemed like the simplest one.

Let me know what you think!