Fallback response SW sample

@kenjibaheux @jakearchibald @slightlyoff @gauntface

Here's another SW recipe, which demonstrates returning a fallback response when the initial request returns an HTTP error response.

You can view a live demo at https://rawgit.com/jeffposnick/samples/sw-fallback-response/service-worker/fallback-response/index.html using the current Chrome Canary release.
I think the pseudo-standard for promise indenting is:

```js
fetch(url).then(function(response) {
  return response.json();
}).then(function(data) {
  console.log(data);
}).catch(function(err) {
  console.log(err);
});
```

Or for longer lines:

```js
navigator.serviceWorker.register(url)
.then(function(reg) {
  console.log(reg);
}).catch(function(err) {
  console.log(err);
});
```