[TIPC]: Unrecognized configuration command now returns error message

This patch causes TIPC to return an error message when it receives
an unrecognized configuration command.  (Previously, the sender
received no feedback.)

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>