HTTP response should not be 200 when returning error-page

Current behavior: 
When WordPress web site was under high concurrency access, the database connections may reach the limitation of the system resource. WordPress was still returning 200 of HTTP response code when database connection error happens. 

The page with error information as:
Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can't contact the database server at dbhost. This could mean your host's database server is down.

Expected behavior: Returns HTTP code with 500 or other error codes to indicate error happens, when showing error pages.