[Bug 1798618] [NEW] confusing error message with 'do-release-upgrade -d' and Prompt=lts

[Steve Langasek]

Public bug reported:

On a bionic system with Prompt=lts set in /etc/update-manager/release-
upgrades, 'do-release-upgrade -d' fails with:

$ sudo do-release-upgrade -d 
Checking for a new Ubuntu release
Upgrades to the development release are only 
available from the latest supported release.
$ echo $?
1
$

The refusal to upgrade is correct, but the message it gives is incorrect
and gives no hint how to fix.

Since bionic is the latest release listed in https://changelogs.ubuntu.com/meta-release-lts *and* in https://changelogs.ubuntu.com/meta-release-lts-development, a more accurate explanation would be: 
   There is no development version of an LTS release available.
   To upgrade to the latest non-LTS development release, set Prompt=normal.

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Medium
         Status: Triaged

** Affects: ubuntu-release-upgrader (Ubuntu Bionic)
     Importance: Medium
         Status: Triaged


** Tags: bionic rls-dd-incoming

** Changed in: ubuntu-release-upgrader (Ubuntu)
   Importance: Undecided => Critical

** Changed in: ubuntu-release-upgrader (Ubuntu)
   Importance: Critical => Medium

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https://bugs.launchpad.net/bugs/1798618

Title:
  confusing error message with 'do-release-upgrade -d' and Prompt=lts

Status in ubuntu-release-upgrader package in Ubuntu:
  Triaged
Status in ubuntu-release-upgrader source package in Bionic:
  Triaged

Bug description:
  On a bionic system with Prompt=lts set in /etc/update-manager/release-
  upgrades, 'do-release-upgrade -d' fails with:

  $ sudo do-release-upgrade -d 
  Checking for a new Ubuntu release
  Upgrades to the development release are only 
  available from the latest supported release.
  $ echo $?
  1
  $

  The refusal to upgrade is correct, but the message it gives is
  incorrect and gives no hint how to fix.

  Since bionic is the latest release listed in https://changelogs.ubuntu.com/meta-release-lts *and* in https://changelogs.ubuntu.com/meta-release-lts-development, a more accurate explanation would be: 
     There is no development version of an LTS release available.
     To upgrade to the latest non-LTS development release, set Prompt=normal.

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