ospfd: better error messages when not enabled in a vrf

ospfd: better error messages when not enabled in a vrf

before:
```
r1# show ip ospf vrf blue
% OSPF instance not found

r1# show ip ospf vrf default
% OSPF instance not found

r1# show ip ospf
% OSPF instance not found
```

after:
```
r1# show ip ospf vrf blue
% OSPF is not enabled in vrf blue

r1# show ip ospf vrf default
% OSPF is not enabled in vrf default

r1# show ip ospf
% OSPF is not enabled in vrf default
```

Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>