code: function handleMongoWriteConcernError(batch, bulkResult, ordered, err, callback) {
    mergeBatchResults(ordered, batch, bulkResult, null, err.result);
  
    const wrappedWriteConcernError = new WriteConcernError({
      errmsg: err.result.writeConcernError.errmsg,
      code: err.result.writeConcernError.result
    });
    return handleCallback(
      callback,
      new BulkWriteError(toError(wrappedWriteConcernError), new BulkWriteResult(bulkResult)),
      null
    );
  }
docs: handles write concern error
  
  @param {object} batch
  @param {object} bulkResult
  @param {boolean} ordered
  @param {WriteConcernError} err
  @param {function} callback