fix: 500 error on optional non-existent directory
fix: 500 error on optional non-existent directory

1. By definition of MultitierFileSystemFinder
non-existent directory are optional search paths.

2. Insert the *theme* at a natural place,
i.e. before the path postfix matching STATIC_URL.

djaodjin/djaodjin-multitier,djaodjin/djaodjin-multitier