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

[Paride Legovini]

The bug reference is not present in d/changelog, unfortunately.
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https://code.launchpad.net/~andersson123/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/453214
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