Improve error messages for the end-user

[martinRenou]

When not having an environment.yml file around, the error Either prefix or environment_file must be set shows up, not giving any clue to the user what to do next. We should provide solutions like:

"Either a prefix or an environment_file must be provided for jupyterlite-xeus. You should try to create an environment.yml file in the location where you build the JupyterLite site. See https://jupyterlite-xeus.readthedocs.io/en/latest/environment.html for examples."