Error thrown while rendering $error.svelte.

[Leftium]

Describe the bug
SvelteKit throws error when rendering $error.svelte:

TypeError: Cannot read property 'message' of undefined

Logs
See stack trace below.

To Reproduce: https://github.com/Leftium/errorkit

npm init svelte@next
yarn
# edit $layout.svelte so `LoadOutput` returned by `load({ page, fetch, session, context })` always results in error.
# Tried:
# 1. Setting LoadoutOutput.error to both new Error() and string.
# 2. Also tried editing index.svelte.

Expected behavior

Svelte's error template should be rendered.

Stacktraces

Stack trace

TypeError: Cannot read property 'message' of undefined
    at eval (D:\tmp\errorkit\.svelte\dev\components\error.svelte:15:41)
    at Object.$$render (D:\tmp\errorkit\node_modules\svelte\internal\index.js:1369:22)
    at Object.default (/D:\tmp\errorkit\.svelte\dev/generated/root.svelte:57:127)
    at eval (D:\tmp\errorkit\src\routes\$layout.svelte:11:41)
    at Object.$$render (D:\tmp\errorkit\node_modules\svelte\internal\index.js:1369:22)
    at eval (/D:\tmp\errorkit\.svelte\dev/generated/root.svelte:55:122)
    at $$render (D:\tmp\errorkit\node_modules\svelte\internal\index.js:1369:22)
    at Object.render (D:\tmp\errorkit\node_modules\svelte\internal\index.js:1377:26)
    at render_response (file:///D:/tmp/errorkit/node_modules/@sveltejs/kit/dist/ssr.js:377:28)
    at async respond_with_error (file:///D:/tmp/errorkit/node_modules/@sveltejs/kit/dist/ssr.js:2572:10)

Information about your SvelteKit Installation:

Diagnostics

• The output of npx envinfo --system --npmPackages svelte,@sveltejs/kit,vite --binaries --browsers

  System:
  OS: Windows 10 10.0.19041
  CPU: (8) x64 Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
  Memory: 11.14 GB / 23.86 GB
  Binaries:
  Node: 14.16.0 - D:\DropBox\a\nodejs\64v14.16.0\node.EXE
  Yarn: 1.22.4 - D:\dropbox\home.yarn\bin\yarn.CMD
  npm: 6.14.11 - D:\DropBox\a\nodejs\64v14.16.0\npm.CMD
  Browsers:
  Edge: Spartan (44.19041.423.0), Chromium (89.0.774.75)
  Internet Explorer: 11.0.19041.1
  npmPackages:
  @sveltejs/kit: next => 1.0.0-next.74
  svelte: ^3.29.0 => 3.37.0
  vite: ^2.1.0 => 2.1.5

• Your browser: Chrome

• Your adapter (e.g. Node, static, Vercel, Begin, etc...): Node

Severity
Not too severe. It just makes it harder to debug SvelteKit projects, and the error output isn't as nice/graceful.

Additional context
Add any other context about the problem here.

[AviDuda]

This was changed yesterday to support nested error components (commit 1007f67), it looks like for now you need to use the load method as shown here in the example.
Later, this should be available via $app/stores (see #901).