FlowStore: Include exception in warning message

Change-Id: I1ff7797a3c21f5c62e2047fd4959711a2efe75c4
diff --git a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/NewDistributedFlowRuleStore.java b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/NewDistributedFlowRuleStore.java
index b691e80..7d33211 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/NewDistributedFlowRuleStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/NewDistributedFlowRuleStore.java
@@ -413,7 +413,7 @@
                                     master)
                            .whenComplete((result, error) -> {
                                if (error != null) {
-                                   log.warn("Failed to storeBatch: {} to {}", operation, master);
+                                   log.warn("Failed to storeBatch: {} to {}", operation, master, error);
 
                                    Set<FlowRule> allFailures = operation.getOperations()
                                            .stream()