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/server/api/requests/execute_command.rs

@@ -145,7 +145,8 @@ fn apply_edit(
                 let reason = response
                     .failure_reason
                     .unwrap_or_else(|| String::from("unspecified reason"));
-                tracing::error!("Failed to apply workspace edit: {}", reason);
+                tracing::error!("Failed to apply workspace edit: {reason}");
+                show_err_msg!("Ruff was unable to apply edits: {reason}");
             }
             Task::nothing()
         },