code: func BadRequest(w http.ResponseWriter, r *http.Request, err *Error) {
  	handlerMu.RLock()
  	f, ok := handlerMap[http.StatusBadRequest]
  	handlerMu.RUnlock()
  	if ok {
  		r = ctxSetErr(r, err)
  		f.ServeHTTP(w, r)
  	} else {
  		defaultBadRequest(w, r, err)
  	}
  }
docs: // BadRequest logs a 400 error and then returns a 400 response to the client.