[w3c/webcomponents] Steps in element definition that throw an exception should abort (#543)

In [customElements.define](https://html.spec.whatwg.org/#dom-customelementsregistry-define), we should clarity we're supposed to keep invoking `Get(prototype, callbackName)` when one of the get calls or the conversion (in step 13.4.2) throws.  I think we're aborting here immediately but that's not entirely obvious from the text.

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/543