[Merge] ~andersson123/ubuntu-release-upgrader:fix_distupgradequirks_traceback into ubuntu-release-upgrader:ubuntu/main

[Tim Andersson]

https://git.launchpad.net/ubuntu-release-upgrader/commit/?id=390f55a383287396eb2e6ce91d2c6df098bc954a
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https://code.launchpad.net/~andersson123/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/453214
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