router_yyerror: cleanup error messages from implementation details
router_yyerror: cleanup error messages from implementation details

Rewrite the error message to turn symbol names in the actual tokens from
the grammar, such that the user isn't confronted with internal
implementation details.

grobian/carbon-c-relay,grobian/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,grobian/carbon-c-relay,piotr1212/carbon-c-relay