More helpful error message when accessing a not-yet-installed app
Migrated from: https://tracker.clearos.com/view.php?id=2201
Reported by: Marc Laporte
In the process of administering ClearOS (ex.: following a procedure in a wiki page), it's possible to access an administrative page such as https://example.org:81/app/mysql
Instead of getting "404 Page Not Found", if the current user has sufficient rights:
The app you tried to access is not yet installed. Please visit the https://example.org:81/app/marketplace.
If it's not trivial to detect that it's an app page, perhaps just adapt the general 404 page.