Added error reporting to main.cpp, added test for error reporting

I've added basic error reporting to main.cpp. This should ideally be done in a different, rule based way but knowing at which point the parser stopped consuming input is usually incredible helpful. Since writing big-ish grammers for egg becomes a pain without any error info, I've added this reporting style for now. If you are interested I'd be happy if we could get this or a similar feature into your master branch.