code: public MongoBulkWriteException getError() {
          return hasErrors() ? new MongoBulkWriteException(createResult(),
                                                      new ArrayList<BulkWriteError>(writeErrors),
                                                      writeConcernErrors.isEmpty() ? null
                                                                                   : writeConcernErrors.get(writeConcernErrors.size() - 1),
                                                      serverAddress) : null;
      }
docs: Gets the combined errors as an exception
  @return the bulk write exception, or null if there were no errors