[w3c/ServiceWorker] messageerror event should use ExtendableMessageEvent (#1193)

Spec says:

> If this throws an exception, catch it, fire an event named messageerror at destination, using MessageEvent, with the origin attribute initialized to origin and the source attribute initialized to source, and then abort these steps.

but MessageEvent.source cannot contain the source if it is a Client. We should probably use ExtendableMessageEvent instead.

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