Add exception for failure to get network preserver

coriolis/osmorphing/base.py

@@ -586,6 +586,9 @@ class BaseLinuxOSMorphingTools(BaseOSMorphingTools):
 
         net_preserver_class = factory.get_net_preserver(self)
         LOG.info("Using network preserver class: %s", net_preserver_class)
+        if not net_preserver_class:
+            raise exception.CoriolisException(
+                "Could not find any valid static network configuration")
         netpreserver = net_preserver_class(self)
         netpreserver.parse_network()
         LOG.info("Parsed network configuration: %s",