Replace emit-related failwiths with unit in mCP and constraints

src/analyses/mCP.ml

@@ -333,7 +333,7 @@ struct
     in
     let emit = match emits with
       | Some emits -> (fun e -> emits := e :: !emits) (* [emits] is in reverse order. *)
-      | None -> (fun _ -> failwith ("Cannot \"emit\" in " ^ tfname ^ " context."))
+      | None -> (fun _ -> ())
     in
     let querycache = Queries.Hashtbl.create 13 in
     let rec ctx' =

src/framework/constraints.ml

@@ -569,7 +569,7 @@ struct
     (* now watch this ... *)
     let rec ctx =
       { ask     = (fun (type a) (q: a Queries.t) -> S.query ctx q)
-      ; emit    = (fun _ -> failwith "emit outside MCP")
+      ; emit    = (fun _ -> ())
       ; node    = fst var
       ; prev_node = prev_node
       ; control_context = snd var |> Obj.obj