[Merge] ~nteodosio/update-manager:x-proerror into update-manager:ubuntu/xenial

[Nathan Teodosio]

Review: Disapprove

Abandon this as per Robie's comment to https://code.launchpad.net/~nteodosio/update-manager/+git/update-manager/+merge/469201;
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https://code.launchpad.net/~nteodosio/update-manager/+git/update-manager/+merge/469196
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