Re: [push-api] Provide support for subscription association (#182)

One question that came to mind - developers tend to call subscribe() multiple times, also as a means to get an existing subscription, since the API only supports once.

What happens when calling subscribe() with an `applicationServerKey` that differs from the subscription that's currently stored by the UA? Should we throw an error? `InvalidAccessError` sounds most appropriate, but `NoModificationAllowedError` (less common) might also work. Happy to address this in a follow-up.

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https://github.com/w3c/push-api/pull/182#issuecomment-176238989