engine: Add network name to "network used" error message

This to assist users to identify which networks were not created as
part of a batch import operation.

Change-Id: I238d57ac3ae943aed312f45a2e6b7b4978b8f95b
Bug-Url: https://bugzilla.redhat.com/1048752
Signed-off-by: Lior Vernia <lvernia@redhat.com>