Refs #9117 - Ignore unnasigned spectra suring tube calibration.

But also print a warning of which ones were skipped.

Also had to fix some problematic indentation surrounding my changes.
Refs #9097 - Print only one "detectors missing" warning per call.

Also, only print the number of detectors that were missing rather than the
indices of each and every histogram.
Merge remote-tracking branch 'origin/feature/9117_handle_unassigned_spectra_during_calibration'