Thrown exceptions from __layout.svelte load result in plain text error responses

Describe the bug

Throwing an exception (or returning a rejecting promise) from a load function should be equivalent to returning { status: 500, error } (apart from the former also resulting in a call to the handleError hook). However, currently, it results in a plain text error message response from the server, which should be returned only when the __error.svelte component fails to render.

Reproduction

Starting from the demo app, add this to __layout.svelte:

<script context="module">
	export function load() {
		throw new Error("FOO");
	}
</script>

Navigate to any page.

Logs

No response

System Info

System:
    OS: Linux 5.10 Ubuntu 20.04.3 LTS (Focal Fossa)
    CPU: (8) x64 Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz
    Memory: 6.36 GB / 12.32 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Yarn: 1.22.15
    npm: 8.1.4
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.4
    @sveltejs/kit: next => 1.0.0-next.202
    svelte: ^3.44.0 => 3.44.3

Severity

serious, but I can work around it

Additional Information

This is inconvenient because I’d like to use the handleError hook as the sole place for performing certain kinds of error logging, but I’d also prefer to not return a plain text error response to users. This means that currently my __layout.svelte’s load function needs to catch the error and reimplement the handleError handling in a different way (since it has access to load arguments instead of to the request object).

[Rich-Harris]

It’s now possible to (group) your roots and put the root +error.svelte outside the group…

src/routes/
├ (main)
│ ├ foo/
│ ├ bar/
│ ├ etc/
│ ├ +layout.svelte # can safely throw errors in here
│ └ +page.svelte
├ +error.svelte   
└ +layout.svelte   # optional

…which mostly solves this issue. That said, there are always going to be cases where we need to bail out to something (e.g. the +error.svelte can’t render because it references data.undefined.something).

Right now, that’s a plain text response. I think we should make that a more appealing HTML page — something very basic, but nicer than what we currently have — and allow people to create a src/error.html page that overrides it, allowing you to at least render a branded page with a ‘visit our Twitter page for updates’ or whatever.

Unlike +error.svelte, we can guarantee that src/error.html can be rendered without errors.

Changing the plain text error response to HTML would be a breaking change, so I’ve added the label.

[Conduitry]

🤷 I mention in the “Additional Information” section specifically why I’m looking for this behavior. I currently have to duplicate some handleError hook logging logic in the __layout load.