zebra: display rpc error msg to vtysh

zebra: display rpc error msg to vtysh

Zebra's clear duplicate detect command is rpc converted.
There is condition where cli fails with human readable message.
Using northboun's errmsg buffer to display error message to
user.

Testing:

bharat# clear evpn dup-addr vni 1002 ip 2011:11::11
Error type: generic error
Error description: Requested IP's associated MAC aa:aa:aa:aa:aa:aa is still in duplicate state

bharat# clear evpn dup-addr vni 1002 ip 11.11.11.11
Error type: generic error
Error description: Requested IP's associated MAC aa:aa:aa:aa:aa:aa is still in duplicate state

Signed-off-by: Chirag Shah <chirag@nvidia.com>