[w3c/webcomponents] "The problematic position" when Custom Elements throw errors? (#547)

There are two places where Custom Elements [report the exception]:

1. Step 7 of [create an element for a token]
2. Step 1.2.1 of [invoke custom element reactions]

To [report the exception], the spec requires "the problematic position (line number and column number) in the resource containing the script". However, in Custom Elements, UA validates the return value from JS and throws errors, and thus "the problematic position" is ambiguous.

This affects:

1. File name, line number, and column number reported in error event and to console.
2. Whether the script has [muted errors] or not to determine what to [report the error].

We tentatively use the position of the constructor function, but some reviewers pointed out that it is not exactly true and should be clarified in the spec.

Thoughts?

The original discussion was at [crrev.com/2244203002](http://crrev.com/2244203002) in case it helps.

[create an element for a token]: https://html.spec.whatwg.org/multipage/syntax.html#create-an-element-for-the-token
[invoke custom element reactions]: https://html.spec.whatwg.org/multipage/scripting.html#invoke-custom-element-reactions
[muted errors]: https://html.spec.whatwg.org/multipage/webappapis.html#muted-errors
[report the error]: https://html.spec.whatwg.org/multipage/webappapis.html#report-the-error
[report the exception]: https://html.spec.whatwg.org/multipage/webappapis.html#report-the-exception

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