error message: log proper error when S3 bucket has pre-existing data

It might get confusing if multiple applications are writing data to the same S3 bucket. Hence, Parseable server requires an empty and dedicated bucket to work on. 

Currently if the bucket has an existing prefix, error message looks like `Error: test/.stream.json not found`. This is confusing since what we mean here is that `test` prefix shouldn't be there at all. 