Update exception reference

The `GDS::SSO::ControllerMethods::PermissionDeniedException` class has
 been deprecated in favour of the new `GDS::SSO::PermissionDeniedError`
 class. Update to raise the new exception to avoid deprecation warnings.

app/controllers/application_controller.rb

@@ -50,7 +50,7 @@ def ensure_user_can_access_tagathon_tools!
   end
 
   def deny_access_to(subject)
-    raise PermissionDeniedException, "Sorry, you are not authorised to access this #{subject}."
+    raise PermissionDeniedError, "Sorry, you are not authorised to access this #{subject}."
   end
 
   def permission_checker