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.

lib/pundit.rb

@@ -11,6 +11,7 @@ module Pundit
     attr_accessor :query, :record, :policy
   end
   class AuthorizationNotPerformedError < StandardError; end
+  class PolicyScopingNotPerformedError < AuthorizationNotPerformedError; end
   class NotDefinedError < StandardError; end
 
   extend ActiveSupport::Concern
@@ -58,7 +59,7 @@ module Pundit
   end
 
   def verify_policy_scoped
-    raise AuthorizationNotPerformedError unless @_policy_scoped
+    raise PolicyScopingNotPerformedError unless @_policy_scoped
   end
 
   def authorize(record, query=nil)

spec/pundit_spec.rb

@@ -148,7 +148,7 @@ describe Pundit do
     end
 
     it "raises an exception when policy_scope is not used" do
-      expect { controller.verify_policy_scoped }.to raise_error(Pundit::AuthorizationNotPerformedError)
+      expect { controller.verify_policy_scoped }.to raise_error(Pundit::PolicyScopingNotPerformedError)
     end
   end