[FEATURE REQUEST] allsky_overlay.py needs to check for errors reading *fields.json files

[EricClaeys]

If the fields.json and/or userfields.json files have an error, e.g., a missing double quote, allsky_overlay.py will give a misleading error message and quit without displaying the overlay.

allsky_overlay.py needs to check if reading the files worked - if not display a meaningful message and don't log any "No Type for xxxx" messages for the variables in the corrupt file(s).
Instead, the first variable in the files that's on the overlay should be replaced by a "See the WebUI" message.