Do not propose to dist-upgrade without a network connection

When there are dependency issues, update-manager propose to run a dist-upgrade.
If there is no network (no interface set or network disabled) update-manager starts a dist-upgrade and fails with a 'Failed to authenticate' error.

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ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: update-manager 1:0.151.6
ProcVersionSignature: Ubuntu 3.0-1.2-generic 3.0.0-rc3
Uname: Linux 3.0-1-generic i686
Architecture: i386
Date: Mon Jun 27 10:59:38 2011
EcryptfsInUse: Yes
GConfNonDefault:
 /apps/update-manager/check_new_release_ignore=
 /apps/update-manager/first_run=false
 /apps/update-manager/show_details=true
 /apps/update-manager/window_size=(622,690)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, user)
 LANG=en_US.utf8
 LC_MESSAGES=en_US.utf8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to oneiric on 2011-06-26 (0 days ago)