[ServiceWorker] ServiceWorkerRegistration.update() is unclear when uninstalling flag set (#784)

Step 2 of update() says:

> If registration's uninstalling flag is set, abort these steps.

Clearly the intention is to stop immediately.  This is a promise-returning function, though.  I assume we want to reject the promise here.

What error should we reject with?  InvalidStateError?  TypeError?

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https://github.com/slightlyoff/ServiceWorker/issues/784