dir: Emit an error on non-root downgrade attempts
dir: Emit an error on non-root downgrade attempts

For security, only root can downgrade flatpaks in the system
installation. However, the error message explaining this when a normal
user attempts a downgrade was removed in commit 44cf5076f. This commit
adds back the error message.

Since the current behavior is for a downgrade by a non-root user to fail
with the message "No updates", this commit doesn't fix a security issue.
It's purely about usability.

Fixes https://github.com/flatpak/flatpak/issues/1303

Closes: #1366
Approved by: alexlarsson

handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak