updated the lexer:

- provide the predefined ERROR lexer constant, next to the EOF constant, so user action code (and other user code) can use these constants without having to resort to unreadable numeric '2' to indicate the ERROR lexer token.

- made the API more usable for alternative (user defined) error display practices, which replace the default provided way which only works for monospaced/PRE 'ASCII art' display output (such as the shell console).