Handle error thrown by fact

I'm trying to figure out what the Engine does it a Fact function throws an exception. As far as I can tell, there is no event handler for this and it doesn't return the error as the Fact value or anything like that. It also doesn't seem to be caught by either of the existing `success` or `failure` events. Is this supported in some way or does the entire `engine.run()` bomb out?

首先追蹤 engine.run() 經過 Fact 評估以及現有的成功和失敗事件路徑。確定 Fact 函式拋出例外時的預期行為，包括錯誤是否應被傳播或發出；完成表示行為已經確定、實作並由測試涵蓋。

由索引模型根據 Issue 內容生成。