KAFKA-707 Improve error message in the producer when sending data to a partition without an active leader; reviewed by Jun Rao

core/src/main/scala/kafka/producer/async/DefaultEventHandler.scala

@@ -228,7 +228,8 @@ class DefaultEventHandler[K,V](config: ProducerConfig,
    */
   private def send(brokerId: Int, messagesPerTopic: Map[TopicAndPartition, ByteBufferMessageSet]) = {
     if(brokerId < 0) {
-      warn("Failed to send to broker %d with data %s".format(brokerId, messagesPerTopic))
+      warn("Failed to send data %s since partitions %s don't have a leader".format(messagesPerTopic.map(_._2),
+        messagesPerTopic.map(_._1.toString).mkString(",")))
       messagesPerTopic.keys.toSeq
     } else if(messagesPerTopic.size > 0) {
       val currentCorrelationId = correlationId.getAndIncrement