ErrorBoundary Component

[benwis]

Spent some time working on #311 because I need it for various things. I think I've got a fairly straightforward implementation based on Solid's version.

I also have a prototype of a better error handler fallback. Basically I wrapped Axum's ServeDir to return either the file or a prerendered page in Leptos. That should cover not found errors before anything is downloaded.

Then #311 gives an ErrorBoundary, which adds the ability to return Result<T,E> from a Component, and on Error, render an component at the nearest parent . That should allow us to capture Errors from server fns and resources run inside Views

[benwis]

Alright, this should be ready to go. Works in CSR and SSR, although I didn't edit the examples. We can do that later