Add a global error notification to the customizer's main panel.

wpse313731-custom-notification.js

( function( $ ) {

	'use strict';

	wp.customize.bind( 'ready', function () {
		wp.customize.notifications.add(
			'wpse313731-custom-notification',
			new wp.customize.Notification(
				'wpse313731-custom-notification', {
					dismissible: true,
					message: wpse313731.msg,
					type: 'error'
				}
			)
		);
	} );

} )( jQuery );

wpse313731-custom-notification.php

<?php
/**
 * Plugin Name: WPSE313731 Custom Notification
 * Description: Add a global error notification to the customizer's main panel.
 * Plugin URI: https://gist.github.com/timelsass/eac654fbbe67e4c66c515e0aed8cc693
 * Author: Tim Elsass
 * Author URI: tim.ph
 */
 
/**
 * Enqueue CSS and JS for customizer pane.
 */
function wpse313731_custom_notification_enqueue_scripts() {
	$handle = 'wpse313731-custom-notification';
	
	// Register the script.
	wp_register_script( $handle, plugins_url( "$handle.js", __FILE__ ), array( 'customize-controls' ) );
	// Translate the message.
	$translated_msg = array(
		'msg' => esc_html( 'This is a custom message being added to the WordPress Customizer.' ),
	);
	
	// Localize script with translated data.
	wp_localize_script( $handle, 'wpse313731', $translated_msg );
	// Enqueue the script.
	wp_enqueue_script( $handle );
}

add_action( 'customize_controls_enqueue_scripts', 'wpse313731_custom_notification_enqueue_scripts' );