Propagate duplicate IF addr add/del error up to API.

Identify and complain when the same IP prefix is assigned
to two different SW interfaces:

    vpp# set int ip address TenGigabitEthernet6/0/0 1.2.3.4/32
    vpp# set int ip address TenGigabitEthernet6/0/1 1.2.3.4/32
    set interface ip address: Prefix 1.2.3.4/32 already found on
	interface TenGigabitEthernet6/0/0

Change-Id: I1aee1b6a7ddd00d3109a53d8e1b6ce97bf45e372
Signed-off-by: Jon Loeliger <jdl@netgate.com>