zebra: Show session id when printing an error when the client disconnects

zebra: Show session id when printing an error when the client disconnects

Before:

```
2023/06/18 22:00:42 ZEBRA: [VXKFG-8SJRV][EC 4043309121] Client 'bgp' encountered an error and is shutting down.
2023/06/18 22:00:42 ZEBRA: [VXKFG-8SJRV][EC 4043309121] Client 'bgp' encountered an error and is shutting down.
```

After:

```
2023/06/18 22:06:44 ZEBRA: [N5M5Y-J5BPG][EC 4043309121] Client 'bgp' (session id 0) encountered an error and is shutting down.
2023/06/18 22:06:44 ZEBRA: [N5M5Y-J5BPG][EC 4043309121] Client 'bgp' (session id 1) encountered an error and is shutting down.
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>