transaction: Fix "Can't load dependent file" error
transaction: Fix "Can't load dependent file" error

Make the error message translatable, and add ": " between the URL and
the rest of the error.

Closes: #3003
Approved by: mwleeds

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