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