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.
Could this all be moved into a `beforeEach` on the outer `describe`?  As is, I don't like that we define these vars once for all the other tests, then redefine (or rather, create a smaller-scope variable with the same name) for just these few tests.  It would be nice if we could either combine the definitions or better separate them.