Add deployment ids to app locked error message

- Changes the response code from `500: Internal Server Error` to `409: Conflict`
- Fixes #450 

New output:

```
HTTP/1.1 409 Conflict
Content-Type: application/json
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked
```
```json
{
    "deployments": [
        "742dc1b0-d26d-4f75-95f0-808b8ea9eed2"
    ], 
    "message": "App is locked by another operation"
}
```