The underlying issue is that when we encountered an error starting a
conversation (any sort of error, though making `$CODEX_HOME/rules` a
file rather than folder was the example in #8803), then we were writing
the message to stderr, but this could be printed over by our UI
framework so the user would not see it. In general, we disallow the use
of `eprintln!()` in this part of the code for exactly this reason,
though this was suppressed by an `#[allow(clippy::print_stderr)]`.

This attempts to clean things up by changing `handle_event()` and
`handle_tui_event()` to return a `Result<AppRunControl>` instead of a
`Result<bool>`, which is a new type introduced in this PR (and depends
on `ExitReason`, also a new type):

```rust
#[derive(Debug)]
pub(crate) enum AppRunControl {
    Continue,
    Exit(ExitReason),
}

#[derive(Debug, Clone)]
pub enum ExitReason {
    UserRequested,
    Fatal(String),
}
```

This makes it possible to exit the primary control flow of the TUI with
richer information. This PR adds `ExitReason` to the existing
`AppExitInfo` struct and updates `handle_app_exit()` to print the error
and exit code `1` in the event of `ExitReason::Fatal`.

I tried to create an integration test for this, but it was a bit
involved, so I published it as a separate PR:
#9166. For this PR, please have
faith in my manual testing!

Fixes #8803.




---
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* #9166
* __->__ #9011