code: private void handleException(final Call call, final Exception e, final Callback callback) {
      if (call.isCanceled()) {
        callback.onCancellation();
      } else {
        callback.onFailure(e);
      }
    }
docs: Handles exceptions.
  
  <p> OkHttp notifies callers of cancellations via an IOException. If IOException is caught
  after request cancellation, then the exception is interpreted as successful cancellation
  and onCancellation is called. Otherwise onFailure is called.