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?

Start by tracing engine.run() through Fact evaluation and the existing success and failure event paths. Determine the intended behavior when a Fact function throws, including whether the error should be propagated or emitted; done means the behavior is decided, implemented, and covered by tests.

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