print errros only when serializable

Change-Id: I1f9319323ac9cc2a83bf3e1c59af15ceca4ba022
diff --git a/src/com/mirantis/mk/Salt.groovy b/src/com/mirantis/mk/Salt.groovy
index 5151566..8f43643 100644
--- a/src/com/mirantis/mk/Salt.groovy
+++ b/src/com/mirantis/mk/Salt.groovy
@@ -225,7 +225,7 @@
                 } else if (resource.value.result.toString().toBoolean() == false || resource.value.changes || onlyChanges == false) {
                     out[node.key][resource.key] = resource.value
 
-                    if (resource.value.result.toString().toBoolean() == false) {
+                    if (resource.value.result.toString().toBoolean() == false && resource.key instanceof String && node.key instanceof String) {
                         common.warningMsg("Resource ${resource.key} failed on node ${node.key}!")
                     }
                 }