Silence logger messages

[vjon]

This is a generalization of jborg/attic#386

I'm curious as to whether anybody here would find an additional log_level to silence all logging particularly useful? In the example use case presented in the original attic issue, the Repository {} already exists error message could be silenced using --critical. Of course, this would also work to silence the majority of error messages for usage in automated backup scripts/etc.

So then the question becomes whether the --critical flag is sufficient, or whether a --quiet flag would provide enough added benefit to justify inclusion?

[ThomasWaldmann]

I don't think serious log messages should be silenced.

For the specific case of borg init on an existing repo:

just don't run borg init if the repo exists, problem solved.

[enkore]

See #271