add logging if failing to execute on a node

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/action/support/nodes/TransportNodesOperationAction.java

@@ -188,6 +188,9 @@ private void onOperation(NodeResponse nodeResponse) {
         private void onFailure(String nodeId, Throwable t) {
             int idx = index.getAndIncrement();
             if (accumulateExceptions()) {
+                if (logger.isDebugEnabled()) {
+                    logger.debug("failed to execute on node [{}]", t, nodeId);
+                }
                 responses.set(idx, new FailedNodeException(nodeId, "Failed node [" + nodeId + "]", t));
             }
             if (counter.incrementAndGet() == responses.length()) {