Raise more description exception for verify_policy_scoped

* The AuthorizationNotPerformedError is very descriptive of the
  situation when authorization is forgotten. In the case of no
  scoping, it can be a head scratcher.
* This new error type is implemented as a subclass of the current error
  type to prevent regressions in existing code. While this is not ideal,
  it is the simplest solution I could come up with for compatibility.
Merge pull request #166 from iamvery/policy-scoped-exception

Raise more description exception for verify_policy_scoped