- 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.