Do not mark missing/disabled media as a failure in problems

Currently, if a user doesn't have any (active) media configured but is included in a usergroup which is being used in an action this results in a "Failure" in the "Last 20 issues" widget and "Problems" page.

If the particular media type fails (e.g. script execution fails, mail delivery fails) it should be marked as a failure but if a user does not have media ("No media defined for user X") and/or is disabled it shouldn't.