Return JSON for 404, 403, 401, 501, and 500 responses

## Return JSON for 404, 403, 401, 501, and 500 responses

### Acceptance Criteria
1. Those responses return JSON in this format:

```JSON
{
    "message" : "The message goes here"
}
```

### Tasks
- Register error handlers that return JsonResponses in Application\Routes.
- Format 401 response created in OAuthListener appropriately.