tools: autoconfig MulticastDiscovery warns on Nack

refs #3704

Change-Id: Iff86770cf0bf7df7da08ef9b57101c645da0a301
diff --git a/tools/ndn-autoconfig/multicast-discovery.cpp b/tools/ndn-autoconfig/multicast-discovery.cpp
index c1ec0c5..d9eeeb3 100644
--- a/tools/ndn-autoconfig/multicast-discovery.cpp
+++ b/tools/ndn-autoconfig/multicast-discovery.cpp
@@ -147,11 +147,12 @@
 
   m_face.expressInterest(interest,
                          bind(&MulticastDiscovery::onSuccess, this, _2),
-                         bind(m_nextStageOnFailure, "Timeout"));
+                         bind(m_nextStageOnFailure, "HUB Data not received: nacked"),
+                         bind(m_nextStageOnFailure, "HUB Data not received: timeout"));
 }
 
 void
-MulticastDiscovery::onSuccess(Data& data)
+MulticastDiscovery::onSuccess(const Data& data)
 {
   const Block& content = data.getContent();
   content.parse();