Make lagg creation more fault tolerant

• Warn, don't exit, when SIOCSLAGGPORT returns an error.

When we exit with an error during lagg creation, a single
failed NIC (which no longer attaches) can prevent lagg
creation and other configuration, such as adding an IPv4
address, and thus leave a machine unreachable.

• Preserve non-EEXISTS errors for exit status from SIOCSLAGGPORT, in case scripts are looking for it. Hopefully this can be extended if other parts of ifconfig can allow a "soft" failure.

• Improve the warning message to mention what lagg and what member are problematic.

Reviewed by: jtl, glebius
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D15046