Specify a predictable error type for unimplemented Element subtypes

[eladalon1983]

Fixes #55.

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[eladalon1983]

Note that this is currently under produceCropTarget(). Depending on which PR is merged first - this one or #50 - the other one will have to be updated.

[eladalon1983]

During today's Editors' meeting, @jan-ivar and @youennf have been opposed to merging this PR. If you've changed your mind - great, let's merge. Otherwise - could you please explain why? For transparency's sake, I think it would be good to continue this discussion publicly. I know some folks would be interested, e.g. @yoavweiss.

[eladalon1983]

What's the explanation for not merging this PR, btw? Is it the same as #54, or is it different?

I've found this side, which someone is intending to present at the next WebRTC WG interim. @jan-ivar / @youennf, I think it's one of you? Or possibly both? I bears mentioning that the claim I highlighted on this slide, would not have been valid if the present PR were merged.

CC @dontcallmedom @yoavweiss

[alvestrand]

Needs to be updated after merge of #50