code: void postFailure(final DownloadRequest request, final int statusCode, final String errMsg) {
      downloadPoster.execute(new Runnable() {
        @Override public void run() {
          request.downloadCallback().onFailure(request.downloadId(), statusCode, errMsg);
        }
      });
    }
docs: Post download failure event.
  
  @param request download request
  @param statusCode status code
  @param errMsg error message