message: add a backtrace to the error! log when a responder fails to respond
diff: @@ -179,7 +179,11 @@ impl<T> Responder<T> {
  pub(crate) async fn respond(mut self, data: T) {
  if let Some(sender) = self.0.take() {
  if sender.send(data).is_err() {
  - error!("could not send response to request down oneshot channel");
  + let backtrace = backtrace::Backtrace::new();
  + error!(
  + ?backtrace,
  + "could not send response to request down oneshot channel"
  + );
  }
  } else {
  error!("tried to send a value down a responder channel, but it was already used");
  