Comment 11 for bug 1583276

That one is slightly different since it's an exception that is not handled yet. I guess I'll have to hurry landing https://code.launchpad.net/~danilo/charms/trusty/glance-simplestreams-sync/invert-failure-mode/+merge/294635 and even push that against 16.05 (that one makes the charm script retry on *all* failures, instead of only on a known subset that we grew by a couple exceptions already).