Currently we ask the user for confirmation, and only after (try to)
remove, failover or migrate the instance. This doesn't work nicely if
the instance doesn't exist, so we make a query for the instance before
the prompt, which will throw an error in case it doesn't exist.

Side-note: the way the query works today is not really nice. It would be
better if we could query explicitly for a missing instance name, so that
this is done cleaner (explicit check) instead of side-effect (throw
exception). We do add code for this explicit check, except that today it
won't be used actually.

Reviewed-by: ultrotter