Improve message when mirror fails to respond or connection to server is broken in some way (proxy, overload)

[Jean-Baptiste Lallement]

Binary package hint: update-manager

At release time, some mirrors are overloaded and fails with a timeout, the connection can also be broken due to proxy issues.
Currently the message returned to the user is "No 'ubuntu-minimal' available/downloadable after sources.list rewrite+update" which is misleading and meaningless. See bug 774505 for example.
This message is also displayed when a proxy doesn't return the expected content.

Update manager should test the connection to the mirror prior to upgrading and display a clear message to the user.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: update-manager 1:0.150
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
Architecture: i386
Date: Mon May 2 14:11:54 2011
EcryptfsInUse: Yes
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, user)
 LANG=en_US.utf8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to oneiric on 2011-02-21 (69 days ago)

[RedSingularity (redsingularity)]

Marking triaged as I have also seen this many times and can confirm. Since this can effect the users overall experience with the system upgrade process, I am going to give it a 'Low' status over 'Wishlist'.

[Jean-Baptiste Lallement (jibel)]

raising priority because it has an impact on a large portion of users