[verification] Spell-out all result cases and priorities.

src/verification.ml

@@ -179,7 +179,12 @@ let combine_prover_answers answers =
     match (acc, r) with
     | Call_provers.Valid, r | r, Call_provers.Valid -> r
     | Invalid, _ | _, Invalid -> Invalid
-    | _ -> acc)
+    | Unknown s, _ | _, Unknown s -> Unknown s
+    | Failure s, _ | _, Failure s -> Failure s
+    | Timeout, _ | _, Timeout -> Timeout
+    | OutOfMemory, _ | _, OutOfMemory -> OutOfMemory
+    | StepLimitExceeded, _ | _, StepLimitExceeded -> StepLimitExceeded
+    | HighFailure, _ -> HighFailure)
 
 let answer_dataset limit config env prover config_prover driver dataset task =
   let dataset_predicate =