Improve no-lamps-so-no-spots error messages

When the backillumination lamps are off, mcsActor exposures eventually fail with something like:

text="command failed: IndexError('index 0 is out of bounds for axis 0 with size 0') in moveToHome() at /software/mhs/products/Linux64/ics_cobraCharmer/1.4.9d/python/ics/cobraCharmer/cobraCoach/cobraCoach.py:1091"

This is alarming and confusing. That Exception should be caught and a better message issued.