Improve error handling in netmask change

Improve error handling in netmask change

- check if the master IP with the old netmask is up before attempting to
  change the netmask (to avoid a failed change netmask resulting in an
  undesired activation of the master IP);
- improve error messages of the backend function;
- in case of error, report the problem but otherwise change the cluster
  master_netmask parameter;
- remove duplicate error feedback.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>