Basically, if you create a PHP file named db-error.php and put under /wp-content/, you’ll get what db-error.php will have. Here is my template on CodePen.
This is example :
<?php
header('HTTP/1.1 503 Service Temporarily Unavailable');
header('Status: 503 Service Temporarily Unavailable');
header('Retry-After: 3600'); // 1 hour = 3600 seconds
mail("[email protected]", "Database Error", "There is a problem with teh database!", "From: Example.COM");
?>
<!DOCTYPE HTML>
<html dir="ltr" lang="en-US">
<head>
<title>503 Service Temporarily Unavailable</title>
<style type="text/css">
h1, p {
font-family: Helvetica, sans-serif;
font-size: 24px;
color: #333;
}
p {
font-size: 14px;
}
</style>
</head>
<body>
<h1>Uff, you came and our server started to sink!</h1>
<p>You did nothing wrong. We are sad but our server's condition is just bad out of load. Please check back after sometime.</p>
</body>
</html>