@Override
  public void onCompletion(Long result, Exception exception) {
    if (content != null) {
      content.release();
    }
    this.result = result;
    this.exception = exception;
    callbackReceived.countDown();
  }
Stores any exception thrown by the operation for which this was set as callback.
If there was no exception or if this was called before callback is received, it will be null.
ambry-rest/src/test/java/com/github/ambry/rest/NettyResponseChannelTest.java