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.
moreso being used as a "quit processing" which is reasonable; however, I generally like to move the return to its own line in that case

```js
if (err || data.Code) {
  cb(new Error('...'));
  return;
}

if (data.Contents.length != 0) {
  cb();
  return;
}

cb(null, file);
```