Fix client-side load error handling

[Rich-Harris]

I don't think there's an issue for this, but it's an annoying bug that has been wrecking productivity. Basically, if an error was encountered in a load function, any props returned from the root layout's load would be unaccounted for when the page hydrates, which triggers the svelte-hmr error overlay (which on a separate note, I think we should get rid of, since it should be covered by Vite's own error handling. cc @rixo, though I'll probably add a separate issue for this in due course).

This PR mostly amends the client-side logic to mirror the server-side logic, with a couple of other bonus fixes for good measure.

[rixo]

Re HMR overlay: it's a setting to toggle in the Vite plugin. Seen with @dominikg, we're going to disable it. Maybe, at a later time, we'll study more thoroughly and see if it would be worthy / doable to funnel those errors it catches through Vite's own overlay (initial discussions we've had around this subject was rather headed in the opposite direction, not trying to catch errors at all).

[Rich-Harris]

@rixo sounds good