Fix logging of error message when an event cannot be indexed (close #237)

Author: istreeter

elasticsearch/src/main/scala/com/snowplowanalytics/stream/loader/clients/ElasticsearchBulkSender.scala

@@ -210,7 +210,7 @@ class ElasticsearchBulkSender(
     error: Option[String],
     record: EmitterJsonInput
   ): Option[EmitterJsonInput] = {
-    error.foreach(e => log.error("Failed to index record in Elasticsearch", e))
+    error.foreach(e => log.error(s"Failed to index record in Elasticsearch: {}", e))
     error
       .flatMap { e =>
         if (