wp_die() function override to redirect to custom error page based on error message

wpmu-wpdie-override-redirect.php

<?php 

// WP_DIE() override - redirect to custom page on specific error

// Note: this is for standard non-Ajax, non-JSON, non-XML requests; 
// for other requests you will need to adjust filters/code
// https://developer.wordpress.org/reference/functions/wp_die/#hooks

add_filter( 'wp_die_handler', 'wpmu_custom_user_wpdie_handler', 11, 1 );


function wpmu_custom_user_wpdie_handler( $message ) {
	
	return 'wpmu_custom_user_wpdie_function';	
	
}

function wpmu_custom_user_wpdie_function( $message, $title, $args ) {
	
	if ( $message == 'The user is already active.' ) {
		wp_redirect( 'URL_TO_CUSTOM_ERROR_PAGE' );
		exit();
	}
	call_user_func( '_default_wp_die_handler', $message, $title, $args );
	

}