500 error after upgrading from 5.0 to 5.4 and adding empty functions.php

I’ve managed to print the error message by adding the following to the functions.php:

function exception_handler($exception) {
    echo "Uncaught exception: " , $exception->getMessage(), "\n";
}

set_exception_handler('exception_handler');

For some reason logs did not include the error details before.
The error is call to undefined function is_blog_installed() which I have no idea why’s that. 😀