dillon1000/react

Turn value block within try/catch into todo (from invariant)

Within a try/catch, every instruction is followed by a maybe-throw terminal. 
This currently breaks the logic in BuildReactiveFunction which tries to 
reassemble the value block, since it isn't expecting the maybe-throw. 
Conceptually the logic should just ignore it — we could even flatten away 
maybe-throw terminals before this pass — but for now since this pattern is rare 
we can just make it a todo.