OCSADV-29: Improved AGS warnings / errors and fixed bug about no guiding feedback update when positions modified in Target Editor.

OCSADV-170 OCSADV-172 OCSADV-175: Updated error messages and fixed bug where guiding feedback wasn't being updated when RA and Dec were changed through TargetEnvironment editor text fields.
Actually I find the `fold` clearer ... with `Option` it's easy for me to mix up `forAll` and `exists`. I always have to stop and think about it. But then again I'm old.