Try - Catch CSOM

I am trying to implement try - catch in CSOM - JavaScript, I have the following code:

        var ctx = new SP.ClientContext.get_current();
        var exScope= new SP.ExceptionHandlingScope(ctx);
        var s= exScope.startScope();

        //try
        var t= exScope.startTry();

        var contactsList = ctx.get_web().get_lists().getByTitle(_title);
        ctx.load(contactsList);
        contactsList.set_description("My Contacts List");
        contactsList.update();

        t.dispose();

        //catch
        var c = exScope.startCatch();

        var listCI = new SP.ListCreationInformation();
        listCI.set_title("Contacts List");
        listCI.set_description("My Contacts List");
        listCI.set_templateType(SP.ListTemplateType.contacts);
        listCI.set_quickLaunchOption(SP.QuickLaunchOptions.on);
        ctx.get_web().get_lists().add(listCI);
        c.dispose();

        //finally
        var f= exScope.startFinally();
        f.dispose();

        s.dispose();
        //Execute
        ctx.executeQueryAsync(onEnsureSuccess, onEnsureFail);

I am defining exeption scope, and try scope to make sure the list exists, if it doesn't a catch will be thrown and it will create a list, so I am handling all errors on the server. What happens is i have an error in onEnsureFail that the list exists, should I get no error? Shouldn't the try block only be executed and the catch not be executed? I don't know what's going on with my code, any help?