I don't think I'd know what to do from this error message—we suggest specifying again, but that will give the same error message.

I think any condition you're requiring (right now, that it starts with s3:// and that it is not a bucket root (has another slash) should have specific error messages that tell you what's wrong, and give what an S3 path should look like (e.g. s3://my-bucket/my-folder).