fix: Don't fail if a to-be archived publish has missing snapshots

[Melkor333]

There was just an ugly stack trace and pyaptly aborted. Now it just says that it wanted to archive a snapshot for a publish which wasn't there. I also added a comment in preparation for an additional feature I'm planning to be able to run pyaptly update CONFIG --create. Also, something we ran into several times. It currently requires a bash script that runs the proper snapshot create; snapshot update; publish create; publish update sequence which is a bit of an annoyance.