Only issue warnings on network errors if warn_only is set

Also, it allows to enter empty passwords at the prompt to just ignore current server and continue with the next.

Long version: We periodically upgrade a lot of servers using fabric, and sometimes some of them are in 'rescue' mode, so the usual credentials don't work and I don't know the rescue password or I want to just ignore the server and continue with the rest. Without this patch I fall on a infinite loop asking for password and can't continue with the other servers.