Better handling of .arcconfig files.

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

Depends on D9697.

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

> arc lint
Usage Exception: Your '~/.arcrc' file is not a valid JSON file.

Parse error on line 18 at column 4: Expected: 'STRING' - It appears you have an extra trailing comma

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9681