Nginx return custom json

custom-error-page

  error_page 400 404 405 =200 @40*_json;

  location @40*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Not Found"}';
  }

  error_page 500 502 503 504 =200 @50*_json;

  location @50*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Unknown Error"}';
  }

Very useful for error-handling on API reverse proxy.
"error_page" directive can be put in specific "location" only, as it supports multiple contexts: http://nginx.org/en/docs/http/ngx_http_core_module.html#error_page