@Override
    public String toString() {
        if (isSuccess()) {
            return "WebSocketUpgradeResult(success)";
        }

        return MoreObjects.toStringHelper(this)
                          .add("fallback", fallbackResponse)
                          .toString();
    }
Returns the fallback {@link HttpResponse} if the upgrade failed.

@throws IllegalStateException if the upgrade was successful.
core/src/main/java/com/linecorp/armeria/server/websocket/WebSocketUpgradeResult.java