Give a more useful error message on image-mode systems

[jlebon]

Some of the image-mode Fedora variants have started shipping dnf5 for layering purposes (see e.g. https://github.com/coreos/fedora-coreos-tracker/issues/1687). However, this is now confusing users trying to use it client-side: https://discussion.fedoraproject.org/t/f41-kinoite-native-container-dnf5-pgp-key-import-error/131946.

We need better error messages, similar to what was done in dnf4, in dnf5:

• https://github.com/rpm-software-management/dnf/pull/2053
• https://github.com/rpm-software-management/dnf/pull/2110
• https://github.com/rpm-software-management/dnf/pull/2112