Raise NotImplementedError for unknown skymap filename

Raise an error when we don't know what the skymap filename is, which currently is when the search group isn't CBC or the pipeline isn't olib or cwb.

requested review from @leo-singer

approved this merge request

enabled an automatic merge when the pipeline for 09c255c2 succeeds

merged

mentioned in merge request !852 (merged)

mentioned in merge request !930 (merged)