Source

File: wp-includes/functions.php

function dead_db() {
	global $wpdb;

	wp_load_translations_early();

	// Load custom DB error template, if present.
	if ( file_exists( WP_CONTENT_DIR . '/db-error.php' ) ) {
		require_once WP_CONTENT_DIR . '/db-error.php';
		die();
	}

	// If installing or in the admin, provide the verbose message.
	if ( wp_installing() || defined( 'WP_ADMIN' ) ) {
		wp_die( $wpdb->error );
	}

	// Otherwise, be terse.
	wp_die( '<h1>' . __( 'Error establishing a database connection' ) . '</h1>', __( 'Database Error' ) );
}