Source

File: wp-includes/class-wp-customize-manager.php

protected function wp_die( $ajax_message, $message = null ) {
		if ( $this->doing_ajax() ) {
			wp_die( $ajax_message );
		}

		if ( ! $message ) {
			$message = __( 'Something went wrong.' );
		}

		if ( $this->messenger_channel ) {
			ob_start();
			wp_enqueue_scripts();
			wp_print_scripts( array( 'customize-base' ) );

			$settings = array(
				'messengerArgs' => array(
					'channel' => $this->messenger_channel,
					'url'     => wp_customize_url(),
				),
				'error'         => $ajax_message,
			);
			?>
			<script>
			( function( api, settings ) {
				var preview = new api.Messenger( settings.messengerArgs );
				preview.send( 'iframe-loading-error', settings.error );
			} )( wp.customize, <?php echo wp_json_encode( $settings ); ?> );
			</script>
			<?php
			$message .= ob_get_clean();
		}

		wp_die( $message );
	}