Add better CORS errors for status=0

[cmdcolin]

The "TypeError: failed to fetch" with status=0 is uninformative. The best thing to do might be to replace this error message with something advising that this is likely a CORS error (some other errors such as bad gzip headers can also cause this)

[cmdcolin]

possibly say to clear cache in error message...

[cmdcolin]

@rbuels mentioned maybe refetch on status=0 with no-cache on