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.
What is this .then for? It does nothing and it would be better to omit it. Does it make sense to move some/all of the previous comment inside this block, if you're seeking to inform people that "hey, you can write code here"?