If a `PhutilProxyException` is thrown by Arcanist, print all error messages.

Summary: `PhutilProxyException` provides the capability to nest exceptions. However, if we throw a `PhutilProxyException` then we currently only display the error message from the top-most exception. Instead, we should print all of the nested exception messages.

Test Plan: Faked an error, saw multiple lines of exception messages.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9697
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
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
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then