js-mode: Add new error constructor

[Amorymeltzer]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/Error

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Personally, I'd use throw new Error more often, but I thought this might be more broadly useful.

I've not updated the big html file, since it seems that hasn't been edited in a while?

[AndreaCrotti]

Ah yes I forgot about the big HTML file, and well that's only generated automatically anyway.
If I automate that process and publish it automatically then we don't have to see it at all in the repo actually.