apt_update `W: Some index files failed to download. They have been ignored, or old ones used instead` shouldn't be ignored

[nobuto-m]

For example in https://bugs.launchpad.net/charm-ovn-chassis/+bug/2070332/comments/19

The charm added an overlay repository to offer a newer version of the software on top of what Ubuntu archive offers (ovn 20.03 in Ubuntu focal vs 22.03 in Cloud Archive repository).

However, the following warnings were recorded but the charm-helper code allowed the charm to move o n and install the software with an old version using the default Ubuntu archive.

2024-08-17 03:38:04 WARNING unit.ovn-chassis-dpdk/0.install logger.go:60 W: Failed to fetch http://ubuntu-cloud.archive.canonical.com/ubuntu/dists/focal-updates/ovn-22.03/InRelease 503 Service Unavailable [IP: 192.168.11.12 8000]
2024-08-17 03:38:04 WARNING unit.ovn-chassis-dpdk/0.install logger.go:60 W: Some index files failed to download. They have been ignored, or old ones used instead.

The expected behavior is the code should error out there and let the helper or charm retry the operation or simply wait an error resolution with an human intervention.