log heartbeat failure in notebook

IPython/frontend/html/notebook/handlers.py

@@ -487,6 +487,7 b' class IOPubHandler(AuthenticatedZMQStreamHandler):'
 
     def kernel_died(self):
         self.application.kernel_manager.delete_mapping_for_kernel(self.kernel_id)
+        self.application.log.error("Kernel %s failed to respond to heartbeat", self.kernel_id)
         self.write_message(
             {'header': {'msg_type': 'status'},
              'parent_header': {},