Modify the src-pos error reporting in parser-tools to better show the token context.
Modify the src-pos error reporting in parser-tools to better show the token context.

Addresses PR 9924.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket