[Merge] lp:~evfool/update-manager/fix665173 into lp:update-manager

[Robert Roth]

Review: Resubmit
Really simple unittest added to check if the error message gets only once in the changelog. The package name is hardcoded for a package I have found that does not support changelogs, but not everyone will have that package though, so it could be replaced with a better package.
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https://code.launchpad.net/~evfool/update-manager/fix665173/+merge/55663
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~evfool/update-manager/fix665173 into lp:update-manager.