ruff server: Important errors are now shown as popups (#10951)

## Summary

Fixes #10866.

Introduces the `show_err_msg!` macro which will send a message to be
shown as a popup to the client via the `window/showMessage` LSP method.

## Test Plan

Insert various `show_err_msg!` calls in common code paths (for example,
at the beginning of `event_loop`) and confirm that these messages appear
in your editor.

To test that panicking works correctly, add this to the top of the `fn
run` definition in
`crates/ruff_server/src/server/api/requests/execute_command.rs`:

```rust
panic!("This should appear");
```

Then, try running a command like `Ruff: Format document` from the
command palette (`Ctrl/Cmd+Shift+P`). You should see the following
messages appear:


![Screenshot 2024-04-16 at 11 20
57 AM](ae430da6-82c3-4841-a419-664ff34034e8)

crates/ruff_server/src/lib.rs

@@ -8,6 +8,8 @@ mod edit;
 mod fix;
 mod format;
 mod lint;
+#[macro_use]
+mod message;
 mod server;
 mod session;