improve notebook interpreter start/stop ux (#2562)

This improves the UX around starting/stopping notebook interpreters, specifically:

1. Displays an error notification if starting a runtime fails.
2. The error notification has a "Retry" button.
3. Fixes an error where shutting down an interpreter between when it was
   started and when it becomes ready would do nothing.

In trying to do the above, I realized that the `notebookRuntimes` map
was responsible for too many things and had become confusing, so I split
that out into "starting" and "active" maps (roughly following
`runtimeSession.ts`).

In my testing, it seems that we don't need to wait until the session is
idle before trying to execute code, so I removed that. Can re-add if it
becomes an issue.

These changes meant that the `NotebookRuntimeData` class was only
tracking the execution order, so I brought that directly into the
`NotebookController` and removed `NotebookRuntimeData`.