Errors reported by iptables during rule loading should cause task to fail

[conorsch]

The iptables load task blithely ignores errors printed to stderr via iptables, for example:

$ sudo /etc/iptables.v4.generated
iptables v1.4.21: unknown option "--dstport"
Try `iptables -h' or 'iptables --help' for more information.
iptables v1.4.21: unknown option "--dstport"
Try `iptables -h' or 'iptables --help' for more information.

The mistake is a typo: --dstport isn't a valid option, and should instead be --dport. Iptables caught that error and reported it, but the misconfiguration didn't percolate up and cause the role execution to fail.