setErrorMessage(inputGroup, message) {
        let errorNode = this.getErrorNode(inputGroup);
        if (errorNode === false) {
            // container for error message doesn't exists, create new
            errorNode = this.createErrorNode();
            this.toggleErrorClass(inputGroup);
            this.insertErrorNode(inputGroup, errorNode)
        }
        // set or update error message
        errorNode.textContent = message;
    }
Creates and includes into DOM error node or updates error message

@param {HTMLElement} inputGroup
@param {String} message