handle failures to find new dark calibrations

[sybenzvi]

Update run_preproc (and possibly run_qproc) to handle new behavior of desispec towards darks. When a suitable dark cal is not found, the new desispec default is to throw an exception rather than quietly fall back to an older dark. This can be overriden by passing fail_on_dark_not_found=False to preproc.

See details in the PR desispec#2162.

[sybenzvi]

Looking at how preproc is called in run_preproc (see run.py), I think we may not need any changes to be compatible with the desispec update. The new desispec will raise an exception if the command-line option --fail-on-dark-not-found is passed. So if we don't change anything, we default to the older behavior that does not raise an exception.