Should raise error when app/routes/index.jsx does not exist

[WesleyYue]

What version of Remix are you using?

1.4.3

Steps to Reproduce

1. Clone https://github.com/WesleyYue/incremental-react-router-to-remix-upgrade-path
2. npm install
3. npm run dev

Expected Behavior

There should be some kind of error that informs the user that they cannot use the catch-all route to render /. This is super non-obvious if migrating from react-router and the user will almost certainly have / as a path under their old react-router implementation. I waste a few hours trying to figure out why my migration didn't work and it's super frustrating to debug. The migration guide does not say anything about needing this index.[jsx/tsx].

Actual Behavior

No error anywhere, just empty rendered by the browser if you follow the migration guide step by step.