update-dlw-krb5-keytabs: Handle missing/empty diskless-workstation-hosts

The "set -e" makes the shell exit with status 1 immediately without any message if the grep in the subshell does not match anything. This in turn makes scripts like gosa-remove-host fail without any error message. Exit gracefully with a message and exit status 0 if diskless-workstation-hosts netgroup is missing/empty.