Added streams to stop files from being uploaded if any files in that fol...

...der exists.

If a file does exist in s3, pipe a null variable to gulp-s3 in which it will fail and generate an error. Once it does, nothing will get uploaded.

This is not final, code still requires cleanup.
I'm not clear on what this test is actually testing.  It tests that `gulp-s3` was called, but that will happen regardless of whether we actually try to write files to `gulp-s3`.  E.g. if you changed `checkS3Repo` to return a stream that just passed files through, would this test fail in that case?