bgpd: [GR] fix mode change vtysh return code

bgpd: [GR] fix mode change vtysh return code

When a bgp neighbor graceful-restart config mode change
is applied, after accepting the config if it does not
take effect instead of throwing vtysh error code,
return the success to vtysh and warn the user.
The debug log is already present at critical code point
where GR failure is seen during config apply.

Ticket: #3761481
Testing Done:

root@tor-1:# vtysh -c 'config t' -c 'router bgp 65564
vrf VRF2' -c 'neighbor 20.1.1.1 graceful-restart'
As part of configuring graceful-restart, capability send to zebra failed
root@tor-1:# echo $?
0

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