Return non-zero on failed "nwkey" command

Return non-zero on failed "nwkey" command

Fail early and exit non-zero immediately instead of indicating success and
possibly carrying the next ifconfig command.

Found at install when wifi interfaces are reset with "-nwid -nwkey -wpa":

	Which network interface do you wish to configure? (or 'done') [bse0] bwfm0
	ifconfig: SIOCS80211NWKEY: Operation not supported by device
	Access point? (ESSID, 'any', list# or '?') [any] 2
	Security protocol? (O)pen, (W)EP, WPA-(P)SK [O]

bwfm(4) currently does not support WEP.

OK stsp