Distribution upgrade (18.10 -> 19.04) silently fails due to available updates

[Matthijs Kooijman]

I'm trying to upgrade from 18.10 to 19.04 by running `update-manager -c` as instructed by https://wiki.ubuntu.com/DiscoDingo/ReleaseNotes. After running it earlier to install pending package updates, I now get a message saying there is a distribution upgrade available (see attached screenshot).

However, when I click "Upgrade..." nothing happens. When running `update-manager -c` from a terminal, I can see that after clicking "Upgrade..." the following is printed and then the process exits:

```
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.
```

Giving that (AFAICS) this tool is supposed to be a GUI application, it seems a bug that this message is only shown on stdout and not in the GUI.

A related problem is that `update-manager -c` does not find any pending updates before deciding to offer an upgrade but does find them when actually trying to upgrade, but that's probably because I messed up some stuff wrt to some ppas and Debian I have installed and apt preferences I set up, since running `aptitude full-upgrade` shows dependency errors. Note that this is probably just my own mess I should clean up, so I'm not intending this to be the subject of this bug report (the lack of a message is). Still, in the interest of understanding and reproducing, here's what `aptitude full-upgrade` said:

```
$ sudo aptitude full-upgrade
The following NEW packages will be installed:
  kicad-demos{a} libglew2.1{a} libpython3.7{a} libpython3.7-minimal{a} libpython3.7-stdlib{a} python3-wxgtk4.0{a}
The following packages will be REMOVED:
  libglew2.0{u} libwxgtk3.0-0v5{u} python-wxgtk3.0{u} python-wxversion{u}
The following packages will be upgraded:
  kicad{b}
1 packages upgraded, 6 newly installed, 4 to remove and 0 not upgraded.
Need to get 32,2 MB/36,0 MB of archives. After unpacking 53,9 MB will be used.
The following packages have unmet dependencies:
 kicad-footprints : Conflicts: kicad-common but it is not going to be installed
                    Conflicts: kicad-common:i386 but it is not going to be installed
 kicad-symbols : Conflicts: kicad-common but it is not going to be installed
                 Conflicts: kicad-common:i386 but it is not going to be installed
 kicad-libraries : Conflicts: kicad-common but it is not going to be installed
                   Conflicts: kicad-common:i386 but it is not going to be installed
 kicad : Depends: libc6 (>= 2.29) but 2.28-0ubuntu1 is installed
         Conflicts: kicad-common but it is not going to be installed
         Conflicts: kicad-common:i386 but it is not going to be installed
 kicad-templates : Conflicts: kicad-common but it is not going to be installed
                   Conflicts: kicad-common:i386 but it is not going to be installed
 kicad-packages3d : Conflicts: kicad-common but it is not going to be installed
                    Conflicts: kicad-common:i386 but it is not going to be installed
The following actions will resolve these dependencies:

     Keep the following packages at their current version:
1) kicad [5.0.2-stable-201811280956+bee76a0~70~ubuntu18.10.1 (now)]
2) kicad-demos [Not Installed]

     Upgrade the following packages:
3) libwxgtk3.0-0v5 [3.0.4+dfsg-4 (cosmic, now) -> 3.0.4+dfsg-8 (testing)]
4) python-wxgtk3.0 [3.0.2.0+dfsg-8 (cosmic, now) -> 3.0.2.0+dfsg-8 (testing)]
5) python-wxversion [3.0.2.0+dfsg-8 (cosmic, now) -> 3.0.2.0+dfsg-8 (testing)]

     Downgrade the following packages:
6) libglew2.0 [2.0.0-6 (cosmic, now) -> 2.0.0-3+b1 (stable)]
```

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: ubuntu-release-upgrader-core 1:18.10.11.6
ProcVersionSignature: Ubuntu 4.18.0-18.19-generic 4.18.20
Uname: Linux 4.18.0-18-generic x86_64
ApportVersion: 2.20.10-0ubuntu13.3
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sat May 4 22:23:41 2019
InstallationDate: Installed on 2019-03-15 (50 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeTermlog: