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.
They all return, making `else`s useless. Makes much better sense to me to just be
```js
if (x) {
  // stuff
  return;
}

if (y) {
  // things
  return;
}

// default
```