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 it's preferable to direct people to chrome://inspect , Service Workers and launch the inspector from there. serviceworker-internals is really for Chrome development. Unfortunately there are a few debugging cases that you really need to use serviceworker-internals, but that's basically a bug.