Fix misleading error message
Fix misleading error message

The error messages here make it appear that values can
be prepended with a '!', but a few lines down we throw
an error if thats the case.

Closes: #2298
Approved by: matthiasclasen

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