message: Call Finish on reader when stream is ended
  * Call Finish on reader when stream is ended
  This should be used to confirm there are no errors.
  * convert grpc status to tfstatus.
diff: @@ -205,7 +205,7 @@ class BigQueryReaderArrowDatasetIterator
  this->response_ = absl::make_unique<apiv1beta1::ReadRowsResponse>();
  if (!this->reader_->Read(this->response_.get())) {
  *end_of_sequence = true;
  - return Status::OK();
  + return GrpcStatusToTfStatus(this->reader_->Finish());
  }
  this->current_row_index_ = 0;
  @@ -286,7 +286,7 @@ class BigQueryReaderAvroDatasetIterator
  if (!this->reader_->Read(this->response_.get())) {
  VLOG(3) << "no data";
  *end_of_sequence = true;
  - return Status::OK();
  + return GrpcStatusToTfStatus(this->reader_->Finish());
  }
  this->current_row_index_ = 0;
  this->decoder_ = avro::binaryDecoder();
  