Error stack improvements

[noelforte]

Caught an issue while working to improve errors in eleventy-plugin-vento. Looks like EleventyErrorHandler's log function loop was assigning to nextRef instead of the originalError key, leading to errors like this:

{% shortcodeThatThrowsAnError %} <!-- => new Error(...) -->

[11ty] Problem writing Eleventy templates:
[11ty] 1. Having trouble rendering njk template ./src/nunjucks.njk (via TemplateContentRenderError)
[11ty] 2. throws an error

instead of

[11ty] Problem writing Eleventy templates:
[11ty] 1. Having trouble rendering njk template ./src/nunjucks.njk (via TemplateContentRenderError)
[11ty] 2. (./src/nunjucks.njk)
[11ty]   EleventyNunjucksError: Error with Nunjucks shortcode `errorThrow` (via Template render error)
[11ty] 3. throws an error

As a related fix, I also added support for the error handler to unwrap cause from the Error object if it's defined, even if cause.originalError doesn't exist. This will be helpful for custom languages like Vento, which attach more errors to the stack via the Error.cause convention.

Would be great to have this in 3.0.1!

(related: noelforte/eleventy-plugin-vento#69)

[zachleat]

This will ship with v3.0.1-alpha.1 thank you!