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.
This test doesn't check whether the files are overwritten.  I.e. if the overwrite protection doesn't work, I think this test would still pass without noticing that files were overwritten.