If mappings are specified (in config) but invalid, CCLib currently just plays the guessing game, which can sometimes cause the wrong mappings to be selected (especially when ForgeGradle is used with the "minecraft.mappings" option)

Suggest the form of this warning? I dialog box? Pull request?

I've been unable to get properly functioning jars out of forgegradle... (for some reason that is probably blatantly obvious, I am getting classnodef's on ASMInit) I can make the PR, but I can't do full testing. (I was thinking Log4J warnings)