Tell Bugsnag of Invalid Submission (closes #1719)

Add a custom "data" tab to the bugsnag message with the current
username, code, and path from the request for additional debugging
resources.

This can be tested locally with the following steps:

1. Register for bugsnag

2. Add BUGSNAG_API_KEY=your_key to the `.env` file at the project root

4. Make a curl request to the local dev server:

```sh
$ curl
  -X POST
  --data '{ "key": "vdv961",
            "code": "some code \n more code ... \n and more code",
            "path": "/some/path"}'
  "localhost:4567/api/v1/user/assignments"
```

The custom data tab would be like in the below image.  Let me know if different info would be appropriate in that tab and I can change what gets included.
![bugsnag_notification](https://cloud.githubusercontent.com/assets/5103414/5365206/d1dfaab8-7fb7-11e4-8242-286f0977a398.png)