Fix: always exit before launching any simulation when -gld is used

lib/sasacore/sasa.ml

-(* Time-stamp: <modified the 18/03/2019 (at 09:21) by Erwan Jahier> *)
+(* Time-stamp: <modified the 20/03/2019 (at 12:09) by Erwan Jahier> *)
 
 (* XXX Je pourrais utiliser Lwt pour rendre step non-bloquant, ce qui
    permettrait d'accelerer la simu sur les machines qui ont plusieurs
@@ -170,7 +170,8 @@ let (make : string array -> t) =
     if args.gen_lutin then (
       let fn = (Filename.remove_extension args.topo) ^ ".lut" in
       if Sys.file_exists fn then (
-        Printf.eprintf "%s already exists.\n" fn; flush stderr
+        Printf.eprintf "%s already exists: rename it to proceed.\n" fn;
+        flush stderr; exit 1
       ) else 
         let oc = open_out fn in
         Printf.fprintf oc "%s" (GenLutin.f pl);