ifconfig: print interface name on SIOCIFCREATE2 error

We have repeatedly gotten reports of unclassified SIOCIFCREATE2 errors
(usually "Device not configured"). This can happen if there is
configuration for interfaces in rc.conf which do not (yet) exist and
we try to configure. I can, e.g., provoke this by configuring wlan
interfaces with their physical interface not installed.
In order to cut support (guesswork) down print the name of the
interface to be configured with the error message.
Hopefully this will help us in the future to improve other configuration
or driver problems.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days