Comment 5 for bug 1607929

One way to get an error message out of update-manager about this is to remove libgtk2-perl, if it is already installed, and run update-manager via a terminal. When a package like software-properties-gtk is installed you'll see the following message in the terminal:

debconf: unable to initialize frontend: Gnome
debconf: (Can't locate Gtk2.pm in @INC (you may need to install the Gtk2 module) (@INC contains: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.24.1 /usr/local/share/perl/5.24.1 /usr/lib/x86_64-linux-gnu/perl5/5.24 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.24 /usr/share/perl/5.24 /usr/local/lib/site_perl /usr/lib/x86_64-linux-gnu/perl-base) at /usr/share/perl5/Debconf/FrontEnd/Gnome.pm line 91.)
debconf: falling back to frontend: Dialog