Allow the ArcanistPhutilLibraryLinter to recover from PHP syntax errors

Summary: Fixes T5577. If the ArcanistPhutilLibraryLinter lints a PHP file that contains a syntax error, it will die horribly. Instead, force it to continue as if nothing was wrong.

Test Plan: Introduced a PHP syntax error. Ran arc lint and made sure the output looked reasonable.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5577

Differential Revision: https://secure.phabricator.com/D9865