Clarify ES publisher warning on first run (#230)

databuilder/publisher/elasticsearch_publisher.py

@@ -150,8 +150,8 @@ class ElasticsearchPublisher(Publisher):
             indices = self.elasticsearch_client.indices.get_alias(self.elasticsearch_alias).keys()
             return indices
         except NotFoundError:
-            LOGGER.warn('Received index not found error from Elasticsearch. ' +
-                        'The index doesnt exist for a newly created ES.')
+            LOGGER.warn("Received index not found error from Elasticsearch. " +
+                        "The index doesn't exist for a newly created ES. It's OK on first run.")
             # return empty list on exception
             return []