message: api: impl Into<Vec<u8>> for ErrorMessage
  So that to_string() intermeidate is not necessary anymore.
diff: @@ -174,6 +174,13 @@ struct ErrorMessage {
  message: String,
  }
  +impl From<ErrorMessage> for Vec<u8> {
  + fn from(msg: ErrorMessage) -> Self {
  + // Safe to unwrap since `ErrorMessage` must succeed in serialization
  + serde_json::to_vec(&msg).unwrap()
  + }
  +}
  +
  pub fn error_response(error: HttpError, status: StatusCode) -> Response {
  let mut response = Response::new(Version::Http11, status);
  @@ -181,8 +188,7 @@ pub fn error_response(error: HttpError, status: StatusCode) -> Response {
  code: "UNDEFINED".to_string(),
  message: format!("{:?}", error),
  };
  - response.set_body(Body::new(serde_json::to_string(&err_msg).unwrap()));
  -
  + response.set_body(Body::new(err_msg));
  response
  }
  