Change the output error message back to use OpenSearchException in the cause chain (#5081) (#5085)

Signed-off-by: Xue Zhou <[email protected]>
(cherry picked from commit d479853)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

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;
         }