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