config: Improve error handling
config: Improve error handling

Use usage_error for commandline-related errors,
as we do elsewhere. And be more specific in the
error messages.

Closes: #2203
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak