Verify YAML configuration after package upgrades

maybe netplan systemd generator should spit out stderr to /run somewhere, and then later have a unit that would cat that file and exit 1, to have a failed unit on boot with errors in the journal, something like netplan-boot-generator-failed.service

Or another approach would be to have a .postinst maintainer script that would copy the current netplan YAML configuration from /{usr/lib,etc,run}/netplan/*.yaml to /tmp/SOME/ROOTDIR/ and then try to apply "/usr/lib/netplan/generate -r /tmp/SOME/ROOTDIR" to check for any parsing issues.