[Merge] lp:~jonas-drange/ubuntu-push/fix-1628327 into lp:ubuntu-push

[Guillermo Gonzalez]

Review: Approve

looks good. but I'm a bit worried about not checking the error returned by systemimage, as it might be a valid error case for a system with system-image.
But I have no idea about the error types returned by the dbus stuff. My only request is to please double check that case.

Thanks.
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https://code.launchpad.net/~jonas-drange/ubuntu-push/fix-1628327/+merge/307159
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