Handle Swift 404 in scrubber

If the image object happened to go missing from the backend, the _delete
function was not catching this exception and caused the app to fall over
constantly.

Also:
* Log the image id rather than uri in the case of UnsupportedBackend to
  avoid printing out credentials.

* Refactor the 'wait for build servers' snippet into a function.
  Jason Kölker's note says the purpose is to wait up to 5 mins, checking
  every 15 seconds, though the code currently was only waiting 15 seconds.
  I have fixed this up as part of the move.