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