Rename writeThrowable to writeException

This commit is a response to upstream change
elastic/elasticsearch@96f283c195.

Original commit: elastic/x-pack-elasticsearch@2e485d4e76

elasticsearch/x-pack/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/action/MonitoringBulkResponse.java

@@ -89,7 +89,7 @@ public class MonitoringBulkResponse extends ActionResponse {
 
         @Override
         public void writeTo(StreamOutput out) throws IOException {
-            out.writeThrowable(getCause());
+            out.writeException(getCause());
         }
 
         /**