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/message.rs

@@ -0,0 +1,62 @@
+use std::sync::OnceLock;
+
+use lsp_types::notification::Notification;
+
+use crate::server::ClientSender;
+
+static MESSENGER: OnceLock<ClientSender> = OnceLock::new();
+
+pub(crate) fn init_messenger(client_sender: &ClientSender) {
+    MESSENGER
+        .set(client_sender.clone())
+        .expect("messenger should only be initialized once");
+
+    // unregister any previously registered panic hook
+    let _ = std::panic::take_hook();
+
+    // When we panic, try to notify the client.
+    std::panic::set_hook(Box::new(move |panic_info| {
+        if let Some(messenger) = MESSENGER.get() {
+            let _ = messenger.send(lsp_server::Message::Notification(
+                lsp_server::Notification {
+                    method: lsp_types::notification::ShowMessage::METHOD.into(),
+                    params: serde_json::to_value(lsp_types::ShowMessageParams {
+                        typ: lsp_types::MessageType::ERROR,
+                        message: String::from(
+                            "The Ruff language server exited with a panic. See the logs for more details."
+                        ),
+                    })
+                    .unwrap_or_default(),
+                },
+            ));
+        }
+
+        let backtrace = std::backtrace::Backtrace::force_capture();
+        tracing::error!("{panic_info}\n{backtrace}");
+    }));
+}
+
+pub(crate) fn show_message(message: String, message_type: lsp_types::MessageType) {
+    MESSENGER
+        .get()
+        .expect("messenger should be initialized")
+        .send(lsp_server::Message::Notification(
+            lsp_server::Notification {
+                method: lsp_types::notification::ShowMessage::METHOD.into(),
+                params: serde_json::to_value(lsp_types::ShowMessageParams {
+                    typ: message_type,
+                    message,
+                })
+                .unwrap(),
+            },
+        ))
+        .expect("message should send");
+}
+
+/// Sends an error to the client with a formatted message. The error is sent in a
+/// `window/showMessage` notification.
+macro_rules! show_err_msg {
+    ($msg:expr$(, $($arg:tt),*)?) => {
+        crate::message::show_message(::core::format_args!($msg, $($($arg),*)?).to_string(), lsp_types::MessageType::ERROR)
+    };
+}