fix: prevent endless SPA 404 loop

[eltigerchino]

fixes #11227

This PR reverts the changes in #11025 that reloaded the page when fetching data failed. Now, it just avoids parsing the response body if it's not JSON and returns the same HttpError with a simplified error message based on the status (we have been doing the same in the server). No test unless we want to setup a new test suite without a root error load function to reproduce the issue.

quick reproduction: https://github.com/eltigerchino/sveltekit-bug-11227

1. Run pnpm i && pnpm dev
2. Navigate to a non-existent page /example
3. See the endless refresh loop

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. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9936f5a

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

[benmccann]

Perhaps we could add a test that would fail without this change? I think we've got a couple test apps that use a fallback page

[eltigerchino]

I ended up creating a new test app because many of the other ones rely on ssr being enabled. This test specifically requires SSR to be disabled in the root layout so that the error page isn't server-side rendered.

[benmccann]

thank you!