[breaking] add error.html

[dummdidumm]

Migration

kit.config.files.template has been renamed to kit.config.files.appTemplate to be more consistent with the new kit.config.files.errorTemplate option.

// svelte.config.js
const config = {
	kit: {
		files: {
-			template: 'src/app.html'
+			appTemplate: 'src/app.html'
		}
	}
};

PR description

This is a static error page that will be rendered by the server when everything else goes wrong
Closes #3068

Implementing this brought up some design questions around how dynamic the error page is. The places I added it previously showed the error message and possibly the stack trace in dev, which is no longer the case. Should we have something like an optional placeholder named %sveltekit.error% which is passed the stringified message, and in dev also the stack trace?

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 2a3e2af

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dummdidumm]

this is somewhat nasty, but the idea could also be reused to test the invalidate() bug I fixed

[Rich-Harris]

from the looks of it this could cause raciness if tests are running in parallel. not suggesting we change anything, just noting it so we know where to look if that starts happening

[Rich-Harris]

We definitely need to include %sveltekit.status% and %sveltekit.message% (though I just remembered that sanitizing error messages in production is still a TODO). I think we should leave stack out of it since it'll always be printed to the terminal (in fact I think the same applies to normal errors — our error handling could use a bit of a tidy-up, separately to this PR)

[dummdidumm]

Made the error page dynamic with respect to status / message