[slightlyoff/ServiceWorker] Never fail for navigation requests (#892)

I suggest to never fail navigation requests. If `fetch` events' `respondWith` fails (passed promise rejected) perform hidden/background request to original URL without informing SW about (SW cannot intercept it).
And for better debugging this situation, write a warning to console about what happened (browsers performed request on its own).

This will prevent web developers (like me) from making mistakes in SW which leads to web site being blocked from loading forever.

I know that browsers has to update SW after 24 hours anyway, bug Firefox had bug when it doesn't and when `caches.match()` fails (rejected) because somehow cache was deleted. Latest fix (`46.0.1`) didn't help, so I suggest to make fundamental change here.

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