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 previously had a logging statement in there, which I then removed without getting rid of the `.then()`. I've removed it now.