When a transaction is aborted implicitly, we currently return different error codes due to different timing. When an operation is running and the transaction gets aborted, we return TransactionAborted, but if the transaction is aborted between operations, the next operation gets NoSuchTransaction. We'd like to unify them to be NoSuchTransaction. This will also be consistent with commands sent after failover, where the new primary expects startTransaction field and returns NoSuchTransaction.