What if a previously-built custom image goes away?

[KevinCLydon]

Currently, if a custom image is built and recorded in the database and then that image disappears for some reason (expires, is manually deleted, etc.), runs that attempt to use that image will just fail and nothing else will be done about it. We should make a change to account for this, either by verifying the existence and accessibility of images before running or identifying that that is the cause of the issue when a run fails. In either case, the image should be built again (if possible) and the run resubmitted.