Stop hardcoding the YACC error message

Summary:
The exact syntax of the error varies with YACC version.

Ref D17188#204123.

Test Plan: Unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17189