Catch focal plane front illumination lights on during exposures

[djschlegel]

If the lights that front-illuminate the focal plane for FVC pictures (installed ~summer 2021?) are left on, they severely contaminate both the GFA images and the spectrographs. The GFAs are saturated. The spectrographs have contaminating blue light like the spectrum below from night 20211128 expid 111175.

NightWatch should trigger an error on any spectra where this is seen (or more generally, any large amount of contaminating light, although here we have the opportunity to diagnose the problem from the spectrum).

Good test cases are the evening cals for night 20211128, expid 111164 - 11197.

[sbailey]

This is a duplicate of #213 (but two reports for same feature are an upvote for its importance!). I'll copy this test to that issue and close this one.