Clarify error message when there is a detector count mismatch

[jlashner]

Properly formats error message that occurs when there is a detector count mismatch between manifestdb and obsfiledb and adds some clarifying detail.

This error message quoted in several open issues and discussions:

• https://github.com/simonsobs/site-pipeline-configs/issues/60
• Detmatch satp1_det_match_240220m failing on 15 obs-ids #1078

[mhasself]

It isn't necessarily the case that such a problem happens when expanding ObsFileDb with the first additional det_info. It could occur elsewhere in the det_info construction chain. So I think more generic "det_info had n_dets={}; after merge has n_dets={}" would be appropriate.

These IncompleteMetadata errors are always caught and re-raised with additional context (so to speak) -- so the actual "metadata" entry that caused the conflict is included in the traceback. (As in the #1078 traceback)

[jlashner]

Sure, that broader error message makes sense to me! I modified it to match your wording.