Return 404 on non-existing resources

Summary:
The fallback route handler is executed also for all public resources
that do not exist. Returning 404 is better than the usual (heavy) view.

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4962