Fix max_bucket test by disallowing partial results (elastic#41959)

The Max Bucket test can potentially return a partial response,
where one of the shards suceeds but another fails due to the max_bucket
setting.  In the case of a partial failure, the status code is 200 OK
since some results were returned (with failures listed in the body).

This makes the yaml test fail since it is expecting a 4xx/5xx failure
when catching exception messages.

We need to disallow partial results so that the entire query fails
and we can check for the max_bucket failure.

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/240_max_buckets.yml

@@ -89,6 +89,7 @@ setup:
       catch: /.*Trying to create too many buckets.*/
       search:
         rest_total_hits_as_int: true
+        allow_partial_search_results: false
         index: test
         body:
           aggregations:
@@ -106,6 +107,7 @@ setup:
       catch: /.*Trying to create too many buckets.*/
       search:
         rest_total_hits_as_int: true
+        allow_partial_search_results: false
         index: test
         body:
           aggregations: