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