[w3c/ServiceWorker] Using registrations that failed to install a ServiceWorker (#1491)

Suppose there's not a registration for `scope` and we run the following:
```
const scope = "./";
navigator.serviceWorker.register("/path/doesnt/exist", { scope });
const registration = await navigator.serviceWorker.getRegistration(scope);
```

Registering a ServiceWorker with an invalid script URL will first place its associated service worker registration in the scope to registration map ([step 6 of Register](https://w3c.github.io/ServiceWorker/#register-algorithm)), but this service worker registration is later evicted from the scope to registration map when [Update fails to fetch the script](https://w3c.github.io/ServiceWorker/#update-algorithm). This all happens before the `navigator.serviceWorker.register` promise settles, but if we don't wait for it to settle (like the snippet above), `getRegistration`/`getRegistrations` can possibly resolve with a "bad" registration (for lack of a better term).

Calling [`update`](https://w3c.github.io/ServiceWorker/#service-worker-registration-update) on one of these bad registrations throws, because there won't be a newest worker (step 3), but calling [`unregister`](https://w3c.github.io/ServiceWorker/#navigator-service-worker-unregister), according to the spec, will work fine because it only uses the scope URL of the service worker registration. This seems inconsistent; should there be a way to deal with such registrations or prevent this situation from occurring?

@asutherland

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