Properly handle "malformed" COMBINE archives

[agarny]

For instance, legacy OpenCOR will generate a message that reads:

/Users/Alan/Desktop/Zhang, Holden, Kodama, Honjo, Lei, Varghese, Boyett, 2000.omex
    Information: only COMBINE archives with one master file are supported.

Zhang, Holden, Kodama, Honjo, Lei, Varghese, Boyett, 2000.omex.zip