setErrorMessage(form_group_el_or_id, msg) {
        if (typeof form_group_el_or_id === 'string') {
            var form_group = document.getElementById(form_group_el_or_id);
        } else {
            var form_group = form_group_el_or_id;
        }

        var error_container = this.getErrorContainer(form_group);

        if (error_container === null) {
            // container for error msg doesn't exists, create new
            var el = this.createErrorContainer();
            this.toggleErrorClass(form_group);
            el.innerHTML = msg;
            this.insertErrorContainer(form_group, el)
        } else {
            // container exists, update msg
            error_container.innerHTML = msg;
        }
    }
Creates and includes into DOM error container or updates error message
@param form_group_el_or_id
@param msg