Change the output error message back to use OpenSearchException in th…

…e cause chain (#5081) (#5084)

Signed-off-by: Xue Zhou <xuezhou@amazon.com>
(cherry picked from commit d479853)

server/src/main/java/org/opensearch/OpenSearchException.java

@@ -603,7 +603,7 @@ public static void generateFailureXContent(XContentBuilder builder, Params param
                 }
                 t = t.getCause();
             }
-            builder.field(ERROR, ExceptionsHelper.summaryMessage(e));
+            builder.field(ERROR, ExceptionsHelper.summaryMessage(t != null ? t : e));
             return;
         }