This adds specific handling for functions.php error loading to re-throw
errors wrapped in a more descriptive message, to make it clear the error
is due to an issue in their functions.php file.
Decided to throw and stop, rather than ignore & continue, to be on the
safe side in the event auth-level (or other security level) customizations
have been made via functions.php.
Adds test to cover.
Closes#4504
