public HttpResponse fallbackResponse() {
        if (fallbackResponse == null) {
            throw new IllegalStateException("WebSocket was successfully upgraded.");
        }
        return fallbackResponse;
    }
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