Gracefully exit if S3 path is not provided for commands which requires it

Based on issue : #178 , thanks @brettswift for identifying a flaw for the command `du`. We need to make sure that the S3 path is provided or it fails with a proper error message to stderr.

Start at the Python CLI handling for the `du` command and run it without an S3 path to observe the current failure. Ensure commands that require an S3 path exit gracefully and write a proper error message to stderr, then verify the behavior with the relevant command tests or CLI invocation.

Written by the indexing model from the issue text.