use print_message for our own exceptions too

toplevel.pl

@@ -58,27 +58,18 @@ execute_command(Command) :-
       throw(error(prolog_failure))
     ),
     Exception,
-    print_exception(Exception)
+    print_message(error, Exception)
   ).
 
 
-print_exception(Exception) :-
-  (
-    Exception = error(Error),
-    error_message(Error, Message)
-  ->
-    format('~w\n', Message)
-  ;
-    format('Uncaught exception: ~p\n', Exception)
-  ).
-
-
-error_message(unknown_command(Command), Message) :-
-  format(atom(Message), 'Unknown command: ~p', [Command]).
+prolog:message(error(unknown_command(Command))) -->
+  ['Unknown command: ~p'-[Command]].
 
+prolog:message(error(prolog_failure)) -->
+  ['Prolog failure'].
 
-error_message(prolog_failure, Message) :-
-  format(atom(Message), 'Prolog failure', []).
+prolog:message(error(Exception)) -->
+   ['Uncaught exception: ~p'-[Exception]].
 
 
 check_types(Command, ArgumentTypes, NewCommand) :-