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