Vague error message for invalid sources

[James Paton-Smith]

This is more like a quality-of-life issue. If you have a source configured which is broken (gpg key error, no release information, etc.), update-manager will provide a vague error message that doesn't really help users identify the problem.

See attached screenshot.

Ideally, update-manager should at least display which sources are causing the error to help track down the problem.
The error message also isn't clear that this doesn't prevent you from updating the system. If you click OK you can still upgrade packages from the remaining correctly configured sources.

ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: update-manager 1:24.04.6
ProcVersionSignature: Ubuntu 6.8.0-31.31-generic 6.8.1
Uname: Linux 6.8.0-31-generic x86_64
ApportVersion: 2.28.1-0ubuntu3
Aptdaemon:

Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Mon May 13 09:32:10 2024
DpkgTerminalLog.txt: Error: [Errno 13] Permission denied: '/var/log/apt/term.log'
GsettingsChanges:
 b'com.ubuntu.update-manager' b'launch-count' b'5'
 b'com.ubuntu.update-manager' b'launch-time' b'int64 1715589118'
 b'com.ubuntu.update-manager' b'show-details' b'true'
 b'com.ubuntu.update-manager' b'window-width' b'593'
PackageArchitecture: all
ProcEnviron:
 LANG=en_GB.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
SourcePackage: update-manager
UpgradeStatus: No upgrade log present (probably fresh install)