More informative error messages in prerendering

[Rich-Harris]

Is your feature request related to a problem? Please describe.
If a page errors during prerendering, you see something like this in your terminal:

500 /foo

This isn't very helpful for diagnosing the underlying error. If it happens locally, you can svelte-kit start and navigate to /foo and see what happened, but if it happens in CI it's a pain (and sometimes errors only happen in CI).

Describe the solution you'd like
It would be helpful if this code...

kit/packages/kit/src/core/adapt/prerender.js

Lines 90 to 105 in ca108a6

	const rendered = await app.render(
	{
	host: config.kit.host,
	method: 'GET',
	headers: {},
	path,
	body: null,
	query: new URLSearchParams()
	},
	{
	local: true,
	dependencies,
	only_render_prerenderable_pages: !force,
	get_static_file: (file) => readFileSync(join(config.kit.files.assets, file))
	}
	);

...somehow gave access to the underlying error. It probably shouldn't be on the response directly, but it could maybe be passed back via a side-channel, a la dependencies.

Describe alternatives you've considered
None

How important is this feature to you?
Nice-to-have