Description

When a formatter is unavailable, POST /format fails with 404 here, but the error isn't shown to the user, instead it shows this:

Reproduce

1. add a nonexistent formatter, e.g. "asdf" in jupyterlab settings
2. invoke formatter
3. see error

Expected behavior

I'd expect the error message to be shown, which would help point the way to what's wrong in my configuration.
Presumably need to handle the error format here.

On the server side, I'd recommend sending a JSON error response, rather than a plain-text one. That should be easier for clients to handle consistently.

Context

• jupyterlab 4.2.4
• jupyterlab_code_formatter 3.0.2