If the readConcern specified on the first statement fails to parse, we return an error, but leave the transaction open and _txnResourceStash empty. When we receive another operation in the same transaction, we find that _txnResourceStash is empty and abort the transaction, returning NoSuchTransaction. It would be preferable to immediately abort the transaction when readConcern fails to parse. Similarly for other errors encountered prior to unstashTransactionResources().