make the listFiles method return files in sub-directories. 

This is to facilitate usage like it was S3.
The local file system should only be being used for local dev work.  The log will contain all the info for the developer. And, S3 won't give them squat in the production system.  Throwing here isn't going to give them much, except make them handle exceptions for cases that will never happen in production. That doesn't seem desirable at all.