Comment 5 for bug 1785240

Updated Bionic from proposed and have the 3.28.1-0ubuntu4.18.04.4 version of GNOME Software installed:

$ apt-cache policy gnome-software
gnome-software:
  Installed: 3.28.1-0ubuntu4.18.04.4
  Candidate: 3.28.1-0ubuntu4.18.04.4
  Version table:
 *** 3.28.1-0ubuntu4.18.04.4 500
        500 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     3.28.1-0ubuntu4.18.04.3 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
     3.28.1-0ubuntu4 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

I clicked install on a snap, then hit cancel at the polkit prompt. It correctly showed the error message in GNOME Software.

Verification done.