Throw 404 error when the page can't be found

Problem/Motivation

If the config page has never been saved, the controller will display "no page".
I think it would be better to return a 404 error instead.

Steps to reproduce

Proposed resolution

The controller could throw a NotFoundHttpException, Drupal will catch the exception and display the 404 eror page.

Remaining tasks

User interface changes

API changes

Data model changes