Validate custom installation IDs
Validate custom installation IDs

We want to avoid unnecessary confusion and complications,
so we rule out IDs that are problematic because they will
clash with the default installations.

At the same time, make the error messages for parsing
custom installations more informative.

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