Update README with proper status code handling

[guimeira]

This example made me spend the last couple of hours trying to figure out why on earth my catch block wasn't being executed when my server returned 404, but according to issue #267, this is the expected behavior. This pull request modifies the example to address this.

[Traviskn]

Thanks!

[mklb]

Nice. But I am still figuring out what to do when downloading directly to a path like:

RNFetchBlob
    .config({
      path : XXXX
    })
    .fetch('GET', URL, {
      ...
    })

Would be nice to somehow catch everything that is not 200. currently if my API responds with a 400 and JSON for example the JSON respond gets saved to that location. Which is not the file I wanted of course ;) I do not want to save anything in that case.