Follow-up from "Bschubert/backport log missed cache"

The following discussion from !752 (merged) should be addressed:

• @tristanvb started a discussion:

  I realize this is already in master, and is probably the best thing to do for bst-1.2 branch now that we're so close to release, however, this has problems and should ideally be done differently, but it will require some additional support from the core.

  The main problem I can see here is, if all of the artifact servers contacted did not find the artifact, then the entire task should be marked as SKIPPED.

  SKIPPED also usually indicates that an element in a queue is SKIPPED, which should cause the summary and status bar to show the element as skipped as well.

  This patch will only emit SKIPPED messages, but will not treat the element as SKIPPED in the summary.

I think an interesting approach to resolving this properly is to add some semantic for the queues to raise a special exception in their processing method, allowing the main job harness to treat the entire task as SKIPPED accordingly and bookkeep it properly.