Check that instance exists before confirm. queries

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