Update to rocket 0.5.0 + correct http status code 422

Thanks to @kije for pointing out the breaking changes in rocket and to @huangjj27 for pointing out that the status code 422 exists and implementing the needed changes

The HyperText Transfer Protocol (HTTP) 422 Unprocessable Content response status code indicates that the server understands the content type of the request entity, and the syntax of the request entity is correct, but it was unable to process the contained instructions.