nginx maintenance page that is JSON friendly

page.conf

  if (-f $document_root/system/maintenance.html) {
    return 503;
  }

  error_page 503 @maintenance;
  location @maintenance {
    internal;
    if ($http_accept ~ json) {
      return 503 "{}";
    }
    rewrite  ^(.*)$  /system/maintenance.html last;
    break;
  }