fix: don't start debugger on 404s

[dummdidumm]

Also directly trigger native navigation for external links to prevent wrong invocation of handleError
closes #9399

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[opensas]

Since now SvelteKit's goto seems to recognize external URLs, can we safely tell users to use goto for internal and also external URLs?

In that case, we should update goto documentation, removing the last sentence (which IMO it's a great plus, one thing less for developers to care about)