Generic decoder fallback not recognizing when needed

[k4kfh]

There is a bug where if you have a decoder in a certain family, Zephyrcab thinks all decoders in that family are supported. For example, I have one LokSound decoder that is supported, and one that isn't (and thus should use the generic generic decoder object). Unfortunately it sees the ESU LokSound family as being supported, and then only discovers that's not the case with a nasty undefined error a little down the line. Easy to fix.