Don't choke if IPv6 rules fail.

On networks with a low MTU, these IPv6 commands may fail because v6 requires a
minimum MTU of 1280. Ignore the failures, since these networks are clearly meant
to be IPv4 only.

Bug: 15466895
Change-Id: Ie8f24f6e7a316633fb60661a07a3e715b6ab6add