fix: handle redirect thrown from root layout load when client-side navigating to a non-existent page

[eltigerchino]

fixes #11099

There is currently no client-side error handling logic for when we run the root layout load function from rendering the root error page. This catches and handles the unhandled thrown Redirect.

Note: we should be displaying a static fallback page for any other type of error but I'm not sure how to do that client-side (the function already exists but only for the server).

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-12005-svelte.vercel.app/

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