[whatwg/dom] Use "NotSupportedError" when trying to attachShadow() twice (#761)

Discussed in #760. This makes the various cases less distinguishable,
which is desirable. With this change, all three cases behave the same
between:

- A custom element with disabled-shadow
- A custom element with a shadow tree
- A built-in element on the blocklist
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/761

-- Commit Summary --

  * Use "NotSupportedError" when trying to attachShadow() twice

-- File Changes --

    M dom.bs (2)

-- Patch Links --

https://github.com/whatwg/dom/pull/761.patch
https://github.com/whatwg/dom/pull/761.diff

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