do-release-upgrade incorrectly reports no upgrade available if it cannot connect to changelogs.ubuntu.com:443

[Simon Déziel]

We use apt-cacher-ng configured as apt proxy on every machine and when trying to upgrade a Xenial machine, I got this:

# do-release-upgrade
Checking for a new Ubuntu release
No new release found.

Running tcpdump showed that do-release-upgrade tried to reach changelogs.ubuntu.com:443 with a proxy CONNECT through our apt-cacher-ng. Our apt-cacher-ng returned: "403 CONNECT denied (ask the admin to allow HTTPS tunnels)".

do-release-upgrade should not ignore those errors silently, it should report whatever failure it received.

# lsb_release -rc
Release: 16.04
Codename: xenial

# apt-cache policy ubuntu-release-upgrader-core
ubuntu-release-upgrader-core:
  Installed: 1:16.04.29
  Candidate: 1:16.04.29
  Version table:
 *** 1:16.04.29 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:16.04.25 500
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
     1:16.04.12 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.