Token value in custom error reporting

Hello,

error reporting has always been a pain point for me in the use of flex
and bison, especially in coordinated error handling between the scanner
and the grammar. I am very happy to see a lot of improvements in this
area in the 3.6 release!

Trying to use `%define parse.error custom` in a project I realized that
it would be very handy to have the value of the token that caused the
syntax error available in yyreport_syntax_error. Is there a way to get
to it?

Thank you!

Cheers,
Dan