[Merge] lp:~jamesodhunt/ubuntu/vivid/ubuntu-core-upgrader/handle-invalid-removed-file into lp:ubuntu/ubuntu-core-upgrader

[James Hunt]

Tests added.
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https://code.launchpad.net/~jamesodhunt/ubuntu/vivid/ubuntu-core-upgrader/handle-invalid-removed-file/+merge/254372
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