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

In the current spec, when the content request fullscreen on the current fullscreen element, the call would be accepted, and a `fullscreenchange` event is fired without actually changing anything. It doesn't seem to make much sense to me.

`fullscreenchange`, as its name, should indicate that something is just changed. If nothing changes, we should fire `fullscreenerror` instead.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/17