the buildds should not continue a build when the dist-upgrade fails

[Matthias Klose]

this is seen in the current perl transition, in several packages like

texinfo builds on armhf, arm64, ppc64el:
  https://launchpad.net/ubuntu/+source/texinfo/7.1-2build1

libmarpa-r2-perl on armhf, arm64, ppc64el:
https://launchpad.net/ubuntu/+source/libmarpa-r2-perl/2.086000~dfsg-8build2

The dist-upgrade at the start fails, leaving the perl packages at version 5.36, however we are aiming the no-change uploads to go with the newly available 5.38 version.

The current work-around is to identify these wrongly built packages, do another no-change upload, and hope that it then succeeds with the dist-upgrade.

So we rely on the upgrade, or else we have to do 600+ sourceful uploads with explicit versioned dependencies on the perl packages, which is not what we want.

So please let a build fail, when the dist-upgrade fails.

[William Grant (wgrant)]

`apt update` doesn't return non-zero on error unless you set `APT::Update::Error-Mode=any`, apparently. We might want to set that in OverrideSourcesList, where we also set `Acquire::Retries=3`.