ifconfig(8): Make lagg creation more fault-tolerant

ifconfig(8): 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.

Obtained from FreeBSD:
https://github.com/freebsd/freebsd/commit/dde41c97866152e8c7eb1f52f07e3e5ac43b2652
https://reviews.freebsd.org/D15046