Change Elasticsearch timeouts to 429 response instead of 500

When a user passes a timeout to an Elasticsearch request, the user is
telling the system to limit how long the request should take. If that
timeout is breached, it is because the user selected a timeout less than
the time the request needs. Therefore, it is up to the user to select a
long enough time. If they don't, it's on the user to adjust their
request.

Given the above, a breached timeout is an issue with the user request,
and the http response code should refelect that. This commit changes
timeout exceptions to give a 429 response code instead of the default
500.

Author: rjernst

server/src/main/java/org/elasticsearch/ElasticsearchTimeoutException.java

@@ -10,6 +10,7 @@
 package org.elasticsearch;
 
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.rest.RestStatus;
 
 import java.io.IOException;
 
@@ -34,4 +35,10 @@ public ElasticsearchTimeoutException(String message, Object... args) {
     public ElasticsearchTimeoutException(String message, Throwable cause, Object... args) {
         super(message, cause, args);
     }
+
+    @Override
+    public RestStatus status() {
+        // closest thing to "your request took longer than you asked for"
+        return RestStatus.TOO_MANY_REQUESTS;
+    }
 }