If I try to format a file with an error (e.g. invalid syntax), using the built-in fourmolu plugin shows the error message in the notification, but using an external fourmolu simply shows "fourmolu: Internal Error: Fourmolu failed with exit code 3" in the notification, and you have to open the HLS logs to see the stderr

Seems specific to fourmolu:

Perhaps we should put stderr in the InternalError? I think that would match the bundled fourmolu logic. cc @georgefst