suppresses action dispatch routing errors

The errors:
* AbstractController::ActionNotFound
* ActionController::RoutingError
are no longer being logged and are displayed as an error native to the
application.

This prevents bots looking for weaknesses from causing huge amounts of
errors which might trigger application monitoring to raise alarms.

Additionally, the error pages displayed to genuine users can be
presented as part of the application.

For now, this is just a POC to trigger comments. Ideally, this would be put into it's own gem which OpenProject would only mount. 
Prefer single-quoted strings when you don't need string interpolation or special symbols.