Runtime error caused by fromJust in module Dataflow

In line 274 of DataFlow, function arfx uses fromJust assuming that looking up a fact always produces a result. Would you please change the implementation into:

```
arfx arf thing fb =
case lookupFact (entryLabel thing) (joinInFacts lattice fb) of
Nothing -> error $
"Dataflow.hs: arfx => Clueless on label:"
++ (show . entryLabel) thing
Just x -> arf thing x
```

It was hard to spot where the error was raised. Including clues in the error message should help.

Regards

Open Dataflow.hs around line 274 and inspect the arfx implementation and its lookupFact call. Replace the uninformative fromJust failure with the requested explicit error path, then verify that a missing fact reports the label in the runtime error.

Written by the indexing model from the issue text.