Throw an exception instead of exiting
Summary: Ref T5577. The PhutilLibraryMapBuilder should throw a XHPASTSyntaxErrorException exception instead of calling exit.
Test Plan: See D9865.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T5577
Differential Revision: https://secure.phabricator.com/D9867