Add error message for spillover control overmatch

[jacobpwagner]

This just adds a more helpful error message for the case in #44, where the regex matches multiple files. The regex check does not include ^ or $ anchors (possibly appropriately), so in the relatively common case of a channel being a substring of another channel (e.g. "V610" and "UV610"), the match will be ambiguous and the return from grep will be a jagged list instead of a vector.

[jacobpwagner]

Also, one test fails, but it was failing before my changes.