S3 backup exceptions aren't surfaced until the next backup

[eric-weaver]

Affected Southpaw Versions: v0.2.1 to v0.3.1

Issue:
Currently uploading backups to S3 via syncToS3() happen in a background thread. There is an object level Future that we use to block multiple backups from being performed at the same time. Since the result of the syncToS3 function is only evaluated when we call Future.get(), if an exception is thrown during copying backup files to S3, the exception will not be surfaced until the next time waitForSyncToS3() function is called which blocks till the Future is returned. Southpaw will continue processing records until the next backup, but on restart it will rollback (backup.time.s * 2) seconds

Proposed Fix:
Add an ability to add an onError callback to the syncToS3() function that will be fired off when an exception occurs allowing the main thread to listen for this and handle it appropriately.