No GUI error message when upgrade to 20.04 does not work due to outdated packages

Users who are presented with a dialog to upgrade to the next release of Ubuntu and do not click Upgrade right away may have new updates to install on their current release. If that's the case and they click Upgrade the distribution upgrade will silently fail because there is no GUI error message.

Test Case
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1) on a system w/ -updates enabled downgrade a package to the version in -security
2) disable -updates in /etc/apt/sources.list
3) run /usr/lib/ubuntu-release-upgrader/check-new-release-gtk and observe the 'Upgrade' button
4) in a separate terminal enable -updates in /etc/apt/sources.list
5) click 'Upgrade' and watch the upgrade fail with no GUI feedback.

Original Description
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I'm running Ubuntu 19.10 and got a message prompting me to upgrade to 20.04. I clicked the "Upgrade now" (or something similar) button and then nothing happens at all.

Looking in the journal, I see:

jun 22 08:19:00 grubby check-new-release-gtk[27508]: Checking for a new Ubuntu release
jun 22 08:19:00 grubby check-new-release-gtk[27508]: Please install all available updates for your release before upgrading.
jun 22 08:19:00 grubby systemd[4266]: update-notifier-release.service: Main process exited, code=exited, status=1/FAILURE
jun 22 08:19:00 grubby systemd[4266]: update-notifier-release.service: Failed with result 'exit-code'.
jun 22 08:19:00 grubby systemd[4266]: Failed to start Notification regarding a new release of Ubuntu.

I suspect this is the reason nothing happens, but that should really be reported to the user graphically, rather than just in the journal.

ProblemType: BugDistroRelease: Ubuntu 19.10
Package: ubuntu-release-upgrader-core 1:19.10.15.4
ProcVersionSignature: Ubuntu 5.3.0-59.53-generic 5.3.18
Uname: Linux 5.3.0-59-generic x86_64
ApportVersion: 2.20.11-0ubuntu8.9
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Mon Jun 22 08:21:57 2020
InstallationDate: Installed on 2019-03-15 (464 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
PackageArchitecture: allSourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to eoan on 2020-01-30 (143 days ago)
VarLogDistupgradeTermlog: