Log Kafka send errors as warn

Log errors encountered when sending messages to Kafka at warn, rather than
debug, to improve the visibility of errors such as 'RecordTooLargeException.'

Fixes #177

Fixes #179

lib/logstash/outputs/kafka.rb

@@ -275,7 +275,7 @@ def retrying_send(batch)
           result = future.get()
         rescue => e
           # TODO(sissel): Add metric to count failures, possibly by exception type.
-          logger.debug? && logger.debug("KafkaProducer.send() failed: #{e}", :exception => e);
+          logger.warn("KafkaProducer.send() failed: #{e}", :exception => e)
           failures << batch[i]
         end
       end