message: Print error when server exited unexpectly
diff: @@ -89,8 +89,17 @@ impl ServerInstance {
  let (server, watcher) = future::abortable(server::run_with(config, flow_stat, server_state));
  tokio::spawn(async move {
  - let _ = server.await;
  - debug!("server listening on port {} exited", server_port);
  + match server.await {
  + Ok(unexpected_exit) => {
  + error!(
  + "server listening on port {} exited with result {:?}",
  + server_port, unexpected_exit
  + );
  + }
  + Err(..) => {
  + debug!("server listening on port {} killed", server_port);
  + }
  + }
  });
  watcher
  