Better error reporting

[Hans Aberg]

A suggest (for the future): The parser that Bison outputs is given the
capability of issuing better error messages, other than the standard
"Parsing Error".

I think the information is basically there, in the debugging. Some of the
stuff now used only for debugging should then be translated into a better
error reporting mechanism.

(I have not had time to analyze this idea better -- perhaps later I could
come up with a better analysis.)

  Hans Aberg