1148928 - 404 is returned when publishing a nonexistent repo group

Fix publish to return a 404 like we said we do in the [Publish Docs](https://pulp-dev-guide.readthedocs.org/en/latest/integration/rest-api/repo/groups/publish.html)

[BZ-1148928](https://bugzilla.redhat.com/show_bug.cgi?id=1148928)
MissingResource is added here but not used. I am guessing you got rid of its usage, but forgot to remove it. Not a big deal, just better to remove unused imports :)