code: func ServerError(w http.ResponseWriter, r *http.Request, err error) {
  	handlerMu.RLock()
  	f, ok := handlerMap[http.StatusInternalServerError]
  	handlerMu.RUnlock()
  	if ok {
  		r = ctxSetErr(r, err)
  		f.ServeHTTP(w, r)
  	} else {
  		defaultServerError(w, r, err)
  	}
  }
docs: // ServerError logs the error to the Logger, and then responds to the request
  // with a generic 500 server error message. ServerError panics if err is nil.