ElementInternals: The validationMessage attribute's getter throws exception

[EdgarChen]

Align with other constraint validation attribute or method of ElementInternals interface,
which would throw "NotSupportedError" if the target element is not a form-associated
custom element.

Test also expects this.

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• At least two implementers are interested (and none opposed):
  • It already is implemented in Chrome this way.
  • Firefox will behave the same, too.
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/ff0fb2a6a742d2c9a1abcf9723382dac04979ce6/custom-elements/form-associated/ElementInternals-NotSupportedError.html#L17
• Implementation bugs are filed:
  • Chrome: N/A, it already is implemented this way.
  • Firefox: Will implement in https://bugzilla.mozilla.org/show_bug.cgi?id=1556364
  • Safari: N/A

(See WHATWG Working Mode: Changes for more details.)

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/custom-elements.html ( diff )

[EdgarChen]

cc @domenic

[domenic]

Normatively LGTM; seems like a straightforward omission. Let's just tidy up the prose.

If you could fill out the PR template as well that'd be good. (Most boxes should be trivially checked since it already is implemented this way.)

[annevk]

Drive-by nit: the first list item is indented with two spaces, instead of one.