Log exceptions rather than swallow them

[geo2a]

Sometimes we would like to see the raw error messages when a kup command fails. This is very useful on CI, but I do agree that we'd really want to hide that from the users by default.

I propose we log the messages of exceptions at debug level, for example here in the implementation of kup push:

    except Exception as e:
        rich.print('❗ [red]Could not push binaries to cachix!')
        logger.debug(e.msg) # we should add this
        sys.exit(1)