[fix] error rework cleanup

[dummdidumm]

See here if you want to know how to migrate: #6586

• add missing changeset, closes Changeset missing from #6586. Change not in changelog #6664
• enhance docs, closes Update error() and PageError types to include details property for more descriptive error messages #6669
• fix fallback message in client

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
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• Ideally, include a test that fails without this PR but passes with it.

Tests

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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: afdf41d

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

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[AlexRMU]

You can't do:

Sentry.captureException(error, { event: event });

You need to do:

Sentry.captureException(error, {
    extra: { event: event },
});