- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.3
* Operating System version: macOs sierra
* Java version: 1.8

### Steps to reproduce this issue

Sometimes, when debugging connect time issue, we don't know whether the port is listening.  We can check the connected log if the connect timeout occurs.
}
       } else {
            super.userEventTriggered(ctx, evt);
        }
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
            throws Exception {
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
        try {
            handler.caught(channel, cause);
        } finally {
            NettyChannel.removeChannelIfDisconnected(ctx.channel());
        }
    }
    /**
     * build a bad request's response
     *
     * @param request the request
     * @param t       the throwable. In most cases, serialization fails.
     * @return the response
     */
    private static Response buildErrorResponse(Request request, Throwable t) {
        Response response = new Response(request.getId(), request.getVersion());
        response.setStatus(Response.BAD_REQUEST);
        response.setErrorMessage(StringUtils.toString(t));
        return response;
    }
}