nitcc: show the text of the token in NParserError.message

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: add Rejected tokens

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: add tests for rejected tokens

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: reject unknown keywords

Signed-off-by: Jean Privat <jean@pryen.org>