Update `error(4xx)` generic message, `Error: 4xx`

Describe the problem

Currently, SveltKit automatically generates a HTTP error message, if not provided.

https://github.com/sveltejs/kit/blob/22ed677df8ece41fa2ecc4687a0594bff0664f46/packages/kit/src/runtime/control.js#L13

To check if the error message is generated by SvelteKit, regex is necessary.

<script lang="ts">
  import { page } from '$app/stores';
</script>

<!-- Type guard -->
{#if $page.error}
  {@const isGenericMessage = /^Error: [45]\d{2}$/.test($page.error.message)}
  {@const message = !isGenericMessage ? $page.error.message : 'Unknown Error'}
  <h1>{$page.status}</h1>
  <p>{message}</p>
{/if}

If not, the +error.svelte example from the documentation renders this.

<!-- 403: Error: 403 -->
<h1>{$page.status}: {$page.error.message}</h1>

Describe the proposed solution

1. Provide a blank message for distinction. (e.g. Error: 4xx → '')
2. Better documentation for handling message in +error.svelte.

<!-- src/routes/+error.svelte -->

<h1>
  {$page.status}:
  {$page.error.message || 'custom error message, hopefully based on the HTTP status code.'}
</h1>

---

The message that accompanies an error(...) should ideally be understandable (or even actionable) by end users.

While the above statement https://github.com/sveltejs/kit/pull/11623#issuecomment-1889994428 is true,

• Current Error: 4xx 5xx is not understandable for end users.
• Providing a message for every generic error() is cumbersome.

---

To elaborate on the latter, consider this use-case.

To access the user data in server load functions, the event.locals has to be checked every single time.

// src/routes/admin/+page.server.ts
// and many other *.server.ts files.

import { error } from '@sveltejs/kit';

export const load = async ({ locals }) => {
  if (!locals.auth) error(401);
  if (!locals.auth.isAdmin) error(403);
};

Providing a consistent error message every single time error() is called is rather impossible.

The following is an example from the SvelteKit documentation.

import { error } from '@sveltejs/kit';

/** @type {import('./$types').LayoutServerLoad} */
export function load({ locals }) {
  if (!locals.user) {
    error(401, 'not logged in');
  }
  if (!locals.user.isAdmin) {
    error(403, 'not an admin');
  }
}

Alternatives considered

1. https://github.com/sveltejs/kit/pull/11623

Importance

would make my life easier

Additional Information

Non-existing route generates a Not Found message, which is not consistent.

https://github.com/sveltejs/kit/blob/22ed677df8ece41fa2ecc4687a0594bff0664f46/packages/kit/src/runtime/server/respond.js#L499

Start with packages/kit/src/runtime/control.js, then compare the missing-route handling in packages/kit/src/runtime/server/respond.js and the linked error documentation. Check existing tests for generic HTTP errors and routing errors before deciding how the default message should be represented. Done means the behavior is consistent and the +error.svelte guidance explains how to handle an absent message.