Called to retrieve the records affected from a specific batched command,
first argument is the value that was returned by AddToBatch when it
was called for the command.
Default implementation always returns 1, derived classes override for otherwise
otherwise DbConcurrencyException will only be thrown if sum of all records in batch is 0.
return 0 to cause Update to throw DbConcurrencyException
protected override bool GetBatchedRecordsAffected(int commandIdentifier, out int recordsAffected, out Exception error)