Serve 500 error page with a service worker

[AndrewBestbier]

No description provided.

[Roisi]

@AndrewBestbier can you add a description? not sure what this is :)

[AndrewBestbier]

@louis what were we talking about here again?

[asavin]

So - implement a service worker that would do a service worker's "hello world" - serve 500 error page when network is down. We can expand on more features later.