[w3c/webcomponents] The custom element state when constructor thrown (#533)

The [create an element for a token] step 7 says when [create an element] thrown:
> let element be instead a new element that implements HTMLUnknownElement, with no attributes, namespace set to given namespace, namespace prefix set to null, custom element state "undefined", and node document set to document.

Then when this element is inserted, the [insert a node] tries to upgrade this element.

Shouldn't we prevent to upgrade elements whose constructor thrown?

@domenic @dominiccooney 

[create an element for a token]: https://html.spec.whatwg.org/multipage/syntax.html#create-an-element-for-the-token
[create an element]: https://dom.spec.whatwg.org/#concept-create-element
[insert a node]: https://dom.spec.whatwg.org/#concept-node-insert

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