Make run_instance() bail quietly if instance has been deleted

Right now, we make a big mess in the logs if someone deletes an
instance before we get a chance to run it. This cleans that up
to a single warning message.

Fixes bug 1154292

Change-Id: I2de0c2993d6a16d9482878eee4eef10bfd78c3e6
Merge "Make run_instance() bail quietly if instance has been deleted"