[Bug 1891095] [NEW] update-manager fails silently

[Peter Stalman]

Public bug reported:

When I run the "Software Updater" from the menu, and the click
"Upgrade..." it will fail without any notification.

If I run it from the terminal, I get the reason why:

```
$ sudo update-manager
[sudo] password for user: 
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.
```

This is the same error as when I run:

```
$ sudo do-release-upgrade
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.
```

I would expect the GUI should return this error in a dialog box instead
of just exiting silently.

```
$ lsb_release -rd
Description:	Ubuntu 18.04.5 LTS
Release:	18.04
```

```
$ apt-cache policy update-manager     
update-manager:
  Installed: 1:18.04.11.13
  Candidate: 1:18.04.11.13
  Version table:
 *** 1:18.04.11.13 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1:18.04.11 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main i386 Packages
```

** Affects: update-manager (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1891095

Title:
  update-manager fails silently

Status in update-manager package in Ubuntu:
  New

Bug description:
  When I run the "Software Updater" from the menu, and the click
  "Upgrade..." it will fail without any notification.

  If I run it from the terminal, I get the reason why:

  ```
  $ sudo update-manager
  [sudo] password for user: 
  Checking for a new Ubuntu release
  Please install all available updates for your release before upgrading.
  ```

  This is the same error as when I run:

  ```
  $ sudo do-release-upgrade
  Checking for a new Ubuntu release
  Please install all available updates for your release before upgrading.
  ```

  I would expect the GUI should return this error in a dialog box
  instead of just exiting silently.

  ```
  $ lsb_release -rd
  Description:	Ubuntu 18.04.5 LTS
  Release:	18.04
  ```

  ```
  $ apt-cache policy update-manager     
  update-manager:
    Installed: 1:18.04.11.13
    Candidate: 1:18.04.11.13
    Version table:
   *** 1:18.04.11.13 500
          500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
          500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages
          100 /var/lib/dpkg/status
       1:18.04.11 500
          500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
          500 http://ca.archive.ubuntu.com/ubuntu bionic/main i386 Packages
  ```

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