replace `%sveltekit.message%` with `%sveltekit.error.message%`

[Rich-Harris]

Describe the problem

As of #6586 we have a standard way of dealing with errors in a SvelteKit app. It would make sense if we respected the App.PageError shape in src/error.html.

Describe the proposed solution

Change %sveltekit.message% to %sveltekit.error.message%. We could include other properties on the error object, but there are some caveats here (we can't allow any way for src/error.html to blow up during rendering) so for now we should probably just make that change, and worry about surfacing %sveltekit.error.code% or whatever as a post-1.0 thing

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response

[msigwart]

worry about surfacing %sveltekit.error.code% or whatever as a post-1.0 thing

Are there plans for this already? Would be great to be able to show some other error props in the error.html.