fix exception while trying to throw DependencyFailed because peer_object has no pk
diff --git a/xos/openstack_observer/syncstep.py b/xos/openstack_observer/syncstep.py
index 77a8c17..fc1a6ec 100644
--- a/xos/openstack_observer/syncstep.py
+++ b/xos/openstack_observer/syncstep.py
@@ -91,7 +91,7 @@
                 if (obj.backend_status!=failed.backend_status):
                     obj.backend_status = failed.backend_status
                     obj.save(update_fields=['backend_status'])
-                raise FailedDependency("Failed dependency for %s:%s peer %s:%s failed  %s:%s" % (obj.__class__.__name__, str(obj.pk), peer_object.__class__.__name__, str(peer_object.pk), failed.__class__.__name__, str(failed.pk)))
+                raise FailedDependency("Failed dependency for %s:%s peer %s:%s failed  %s:%s" % (obj.__class__.__name__, str(getattr(obj,"pk","no_pk")), peer_object.__class__.__name__, str(getattr(peer_object,"pk","no_pk")), failed.__class__.__name__, str(getattr(failed,"pk","no_pk"))))
 
     def call(self, failed=[], deletion=False):
         pending = self.fetch_pending(deletion)