Fix panic in matchspec parser

[aochagavia]

The matchspec parser contains a call to unimplemented! (see here). For a program that accepts arbitrary matchspecs, that means specific matchspecs could cause the program to unexpectedly crash. I think it'd make sense to extend ParseMatchSpecError with an additional variant for unimplemented bits, to avoid panics.

Would you welcome a PR for this?

[wolfv]

Sounds good to me! Even better would be to implement whatever conda does there :) but getting rid of panics is always good