docs: public HttpServerExchange setStatusCode(final int statusCode) {
          if (statusCode < 0 || statusCode > 999) {
              throw new IllegalArgumentException("Invalid response code");
          }
          int oldVal = state;
          if (allAreSet(oldVal, FLAG_RESPONSE_SENT)) {
              throw UndertowMessages.MESSAGES.responseAlreadyStarted();
          }
          if(statusCode >= 500) {
              if(UndertowLogger.ERROR_RESPONSE.isDebugEnabled()) {
                  UndertowLogger.ERROR_RESPONSE.debugf(new RuntimeException(), "Setting error code %s for exchange %s", statusCode, this);
              }
          }
          this.state = oldVal & ~MASK_RESPONSE_CODE | statusCode & MASK_RESPONSE_CODE;
          return this;
      }