Fix "backup None not yet finished" message
nixops backup-status: Don't show network info

That's what "nixops info" already does.
nixops remove-backup: Make sure there is only one argument

Otherwise you can get "removing backup None".
nixops backup-status: Give a proper error message if a backup doesn't exist

Previously we got: KeyError: '...'
nixops backup-status: Show snapshot IDs

And some bugfixes.