[multus-thick] Suppress error in case of no connection to server

[s1061123]

What happend:
Multus thick case, multus-shim may send error message for DEL in CNI reply when mutlus-shim cannot connect to server. To following CNI spec, DEL command should not return error.

What you expected to happen:
It should be no error in CNI result.

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Multus version
  image path and image ID (from 'docker images')
• Kubernetes version (use kubectl version):
• Primary CNI for Kubernetes cluster:
• OS (e.g. from /etc/os-release):
• File of '/etc/cni/net.d/'
• File of '/etc/cni/multus/net.d'
• NetworkAttachment info (use kubectl get net-attach-def -o yaml)
• Target pod yaml info (with annotation, use kubectl get pod <podname> -o yaml)
• Other log outputs (if you use multus logging)