## 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:
