Stop emitting FATAL log when checking existence of indices

Passing `ignore: 404` to Elasticsearch::Client can stop
FATAL log in addition to stopping exception.

This option is supported since the 5.0.4 and 6.0.0 of `elasticsearch-ruby`.
https://github.com/elastic/elasticsearch-ruby/blob/36d45f0c12f4e9ef96646f269748ad33b43e974c/CHANGELOG.md#600
https://github.com/elastic/elasticsearch-ruby/blob/36d45f0c12f4e9ef96646f269748ad33b43e974c/CHANGELOG.md#504

This swallowing was implemented following exception catching as follows.
elastic/elasticsearch-ruby@9848c97

elasticsearch-model/lib/elasticsearch/model/indexing.rb

@@ -270,7 +270,7 @@ def create_index!(options={})
         def index_exists?(options={})
           target_index = options[:index] || self.index_name
 
-          self.client.indices.exists(index: target_index) rescue false
+          self.client.indices.exists(index: target_index, ignore: 404)
         end
 
         # Deletes the index with corresponding name