[ground_segment] sane error when launching settings without specifying aircraft

sw/ground_segment/tmtc/settings.ml

@@ -80,11 +80,20 @@ let _ =
 
   let ivy_bus = ref Defivybus.default_ivy_bus in
   let acs = ref [] in
-  Arg.parse
+
+  let anon_fun = (fun x -> prerr_endline ("WARNING: don't do anything with "^x)) in
+  let speclist =
     [ "-b", Arg.String (fun x -> ivy_bus := x), (sprintf "<ivy bus> Default is %s" !ivy_bus);
       "-ac",  Arg.String (fun x -> acs := x :: !acs), "A/C name"]
-    (fun x -> prerr_endline ("WARNING: don't do anything with "^x))
-    "Usage: ";
+  and usage_msg = "Usage: " in
+
+  Arg.parse speclist anon_fun usage_msg;
+
+  if List.length !acs = 0 then begin
+    prerr_endline "Error: Specify at least one Aircraft for which to display the settings!";
+    Arg.usage speclist usage_msg;
+    exit 1
+  end;
 
   (** Connect to the Ivy bus *)
   Ivy.init "Paparazzi settings" "READY" (fun _ _ -> ());