Theme: Added handling for functions.php file load error

Theme: Added handling for functions.php file load error

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