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