Error middleware in static

No containers, file config only. I see that there is an error middleware in the dynamic file configs. Looks straight forward. However, I can't figure out how to redirect if a subomain config file doesn't exist? For my setup, I create a separate config file for each sub domain. I can't change this. I would like to be able show them a nice page that their server is not running and give them instructions on how to start it. I don't see how to do that by the example in the static file on the docs site.