Add #query, #record, and #user properties to Pundit::NotAuthorizedError.

Exceptions raised by #authorize now provide the query (e.g. 'create?') and
record (e.g. an instance of 'Post') that caused the exception to be raised, as
well as the relevant policy (e.g. an instance of 'PostPolicy').

NotAuthorizedError is modified to continue to inherit from StandardError, but
now also has attr_accessor values for :query, :record, and :policy.