docs: @Handler(channels = NetworkChannel.class)
      public void onIoError(IOError event) throws IOException {
          for (Channel channel : event.channels()) {
              if (channel instanceof TcpChannel) {
                  // Error while using established network connection
                  TcpChannel netConnChannel = (TcpChannel) channel;
                  Optional<WebAppMsgChannel> appChannel
                      = netConnChannel.associated(WebAppMsgChannel.class);
                  if (appChannel.isPresent()) {
                      // Error while using a network connection
                      appChannel.get().handleIoError(event, netConnChannel);
                      continue;
                  }
                  // Just in case...
                  pooled.remove(netConnChannel.remoteAddress(), netConnChannel);
                  continue;
              }
              // Error while trying to establish the network connection
              if (event.event() instanceof OpenTcpConnection) {
                  OpenTcpConnection connEvent
                      = (OpenTcpConnection) event.event();
                  Optional<Set<WebAppMsgChannel>> erroneous;
                  synchronized (connecting) {
                      erroneous = Optional
                          .ofNullable(connecting.get(connEvent.address()));
                      connecting.remove(connEvent.address());
                  }
                  erroneous.ifPresent(set -> {
                      for (WebAppMsgChannel chann : set) {
                          chann.openError(event);
                      }
                  });
              }
          }
      }