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.
`hasError` seems unnecessary.  You could use [`fail`](http://chaijs.com/api/assert/) to accomplish the same goal without `hasError`, but I'm still not sure what happens to the error that would be thrown.  I.e. would mocha know to tie the failure to this test?