MutateRowsAttemptCallable when it encounters either RPC or entry errors will resolve its future with a synthetic MutateRowsException. The synthetic error has an INTERNAL status but it marked as retriable. This is done so that the parent RetryableFuture retries the remaining entries. However, once there are no more retriable entries, the MutateRowsException will bubble up to BatcherImpl.Batch#onBatchFailure, which will feed into BatcherStats, which will record the RPC error as INTERNAL