Add the instructions for deletion of backups in S3

When we backup to S3, if we delete a backup taken, we also need to delete the staging directory before being able to backup the same files to any other bucket.

The error message popped up is below :

Backup repository creation failed: populated staging directory used for a remote archive which does not exist, perhaps the remote archive has been removed.

This error message is not very intuitive. Hence, we need to add the instructions for removing staging directory as well as make the error message clear.