Proper syntax errors, plus error recovery

[mascarenhas]

Right now the parser just fails with SyntaxError in case of syntax errors. Error messages should be more informative. The parser should also recover from errors and continue parsing.

[hugomg]

As of today, the parser does a much better job of providing informative error messages. We still don't do any error recovery but I don't think I miss that as much as I missed the better error messages...