protected void renderErrors(Set<ErrorMessage> errors) throws IOException {
        if (errors.isEmpty()) return;

        generator.writeArrayFieldStart(ERRORS);
        for (ErrorMessage e : errors) {
            renderError(generator, e);
        }
        generator.writeEndArray();
    }
Creates a json renderer using a custom executor.
Using a custom executor is useful for tests to avoid creating new threads for each renderer registry.
container-search/src/main/java/com/yahoo/search/rendering/JsonRenderer.java