various fixes for craft handling

- no "UFO lost" popup messages for UFOs attacking a base
- no "select new destination" message for crafts that are following said UFO
- properly delay alien missions in the event of a craft's destruction (not
just on disabling the ufo)
- don't allow "UFO BreakingOff" to override "UFO destroyed"
- end the retaliation if the base is destroyed (one way or another)
- delay spawning of the next UFO for this mission when carrying out an attack
against XCOM bases (no more attacks every 30 minutes)
- move "craft destroyed" checks out of dogfight and into geoscape to avoid cases
where "my interceptor has -105 health"
- add some safety in the UFO destructor to make it send any followers home