No error message on failed update

[David Baumann]

So when I don't have an internet connection for any reason, and run update-manager gui, it pretends to update (apt-get update), but it can't download, because there is no connection.

So it seems to update, and says "all packages are up to date", but after rechecking with a working connection, there are updates available.
I think there should be a warning when run manually.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: update-manager 1:15.10.3
ProcVersionSignature: Ubuntu 4.2.0-18.22-generic 4.2.3
Uname: Linux 4.2.0-18-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jan 15 15:18:33 2016
GsettingsChanges:
 b'com.ubuntu.update-manager' b'show-details' b'true'
 b'com.ubuntu.update-manager' b'window-height' b'1056'
 b'com.ubuntu.update-manager' b'first-run' b'false'
 b'com.ubuntu.update-manager' b'window-width' b'1855'
 b'com.ubuntu.update-manager' b'launch-time' b'1452867015'
InstallationDate: Installed on 2015-04-01 (288 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to wily on 2015-10-24 (83 days ago)