How and what are you uploading ?
If it is a custom report then it is all about you (or the devloper) how to handle failed updates:
- continue with the rest of updates and create message about failure
- stop all further processing and rollback the already updated records.
It also depends what kind of records you are uploading and if you do them all at once or record for record.
I don;t think it will make a difference if you call synchrone or asynchrone, but again it all depends on which records, which BAPI, custom program or standard SAP ?