Show file where syntax error occurred in library map builder

Summary: Fixes T5597. This information is pretty useful but was inadvertently dropped while cleaning up some other stuff.

Test Plan: Ran `arc liberate`, was told which file has the issue.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5597

Differential Revision: https://secure.phabricator.com/D9883