GUI: improve error message when missing data sender public PGP key

It seems that currently the error message displayed by sett-gui when a package cannot be verified due to a missing key is simply:

"Failed to load data package [package_name]: error verifying package - Package verification error"

Missing a key (sender or recipient) is fairly common, so it would be nice if we could give clearer error message, such as:

"Failed to load data package [package_name]: missing public OpenPGP key from data sender (). Please download this key and try again".

Maybe also check whether this same issue is present in the CLI/TUI.