Improve the handling of .arclint files.

Summary: Throw a useful error message when an .arclint file is not valid JSON.

Test Plan:
Modified an .arclint file to be invalid JSON.

> arc lint
Exception
Parse error on line 24 at column 5: Expected one of: 'EOF', '}', ',', ']'
(Run with --trace for a full exception trace.)

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9679