Fix/glue crawler trailing space failure

[asparke2]

Fixes #159

Pull Request Description

Removes any trailing slash from s3_prefix to avoid Athena table creation issues. Also warns if the target s3 directory is empty, which would result in an unexpectedly empty table in Athena.

Checklist

Not all may apply

• Code changes (must work)
• Tests exercising your feature/bug fix (check coverage report on CircleCI build -> Artifacts)
• All other unit tests passing
• Run a small batch run to make sure it all works (local is fine, unless an Eagle specific feature)

[nmerket]

@rajeee @asparke2 The test_upload_files test is failing here. Can one of you look into it?

[asparke2]

@nmerket it fails because now the code checks the s3 directory for files before attempting to run the glue crawler. Because the CI isn't accessing a real s3 bucket, this fails. I assume the solution is to mock an s3 response, but I couldn't figure out how to do this and that's why the PR was sitting.