Nested error components

[Rich-Harris]

We have nested $layout components, but only a top-level $error. It might be useful to support nested error components as well:

In this example, if routes/settings/notifications/$error.svelte errored in load, we'd then try rendering routes/settings/$error.svelte, etc.

[arxpoetica]

Crazy thought -- should error pages belong in layout pages?

[Conduitry]

This was implemented back in #901.