Add a "catch all" route

[Timo Walter]

In ReleaseArtifactController a default / "catch all" action should be added that returns the HTTP status code 404.
Without this action the consumer of the API does not get an error if he tries to call a route that does not exist on the controller (e.g. by misspelling: "/releaseartifact/vesion").

The route attribute for that action:
[Route("{*url}", Order = 999)]