Re: [fullscreen] Requesting fullscreen on the current fullscreen element should be an error (#17)

Step 7.5.2 of `requestFullscreen()` already says "If element is doc's fullscreen element, terminate these subsubsteps. Note: No need to notify observers when nothing has changed."

I think there are two cases to distinguish here:
 1. `requestFullscreen()` is called twice from the same input event handler, perhaps by different listeners or perhaps by broken fallback code that ends up calling both a prefixed and unprefixed API.
 2. A second `requestFullscreen()` call after the fullscreen transition is complete.

In both cases it would make sense, IMHO, to resolve the fulfill the promise, in which case also firing `fullscreenerror` would be odd.

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