public interface ANTLRErrorListener
Defines behavior of object able to handle error messages from ANTLR including
both tool errors like "can't write file" and grammar ambiguity warnings.
To avoid having to change tools that use ANTLR (like GUIs), I am
wrapping error data in Message objects and passing them to the listener.
In this way, users of this interface are less sensitive to changes in
the info I need for error messages.