Handle VM snapshot failures without tracing back in the GUI.

Also don't mark snapshots as vm synced when they are not.
(Yes, failing to snap 1 VM will cause the entire snapshot to
 not be listed, but that is the granularity we have)

Ticket: #6873

Further work is needed though.  It isn't acceptable to silently
fail at snapping VMs if your DR policy depends on this.  The failure
to snap a VM will need to trip the alert system.