Generate HTML error pages for Nexus

Our end of https://github.com/oxidecomputer/omicron/issues/3333

Vite doc on multiple entry points: https://vitejs.dev/guide/build.html#multi-page-app. We would add more entries here:

https://github.com/oxidecomputer/console/blob/baef7af7433f5c394e22f2072ee0280905787c74/vite.config.ts#L25-L29

Definitely a 404 page and a generic something went wrong page, not sure if we need more than that. It would be cool if we could inline the relevant CSS right in the HTML, but there will always be fonts to download, so the page is never going to be totally free of additional dependencies. So making everything be inline is a secondary design goal. But definitely no JS!

They would be automatically included in the console assets tarball, which takes everything generated by Vite and sends it over the fence, so literally all that's required in this repo is making sure the files get generated. On the Nexus side we have to figure out how to serve them when appropriate.

Start with vite.config.ts at lines 25-29 and the linked Vite multi-page app guidance. Add entry points for a 404 page and a generic error page with no JavaScript; confirm Vite generates them and that they are included in the console assets tarball.