Issue #2904358 by bonus, dakku: Moving default settings config to a dedicated settings file instead of readme
Issue #2930381 by sonnykt, bonus: Subscribe to NotFoundHttpException event
Issue #2978684 by bonus: Normalize Fast404 settings names
Issue #2978693 by bonus: Update README and example settings file to explain all configuration options
Issue #2961512 Using the fast404.inc approach throws errors.
Issue #2715585 by bonus, robpowell, naveenvalecha: Add Fast404PathTestCase for testing paths
drupal_set_message() has been deprecated as of Drupal 8.5. This update updates this code to the newer way of doing things.
I have updated the controller to extend the controller base class, so that I can show the message that was being set using the Drupal set message function, using Drupal 8's new messenger class. The messenger functionality has been included using dependency injection.