Nexus VXLAN: raise exception on nxos failure

Need to check for errors from ncc client and
clear the connections on an error with the Nexus so that
the connection is re-established correctly when the Nexus
comes back up.

Co-Authored-By: Carol Bouchard caboucha@cisco.com

Change-Id: Ic9d8bd60b3905a3d53c19057bd61d1356e6b4bc1
Closes-Bug: #1399998