Fix passthrough on non-existing file edge cases

A request does not "passthrough" (when using `opt.passthrough`) when the file is non-existent, providing the folder of that file is an existing regular file.
For example, with a folder containing `image.jpg`, trying to access `image.jpg/thisdoesnotexist` will fire an error instead of passing the request over to the next middleware.
This pull request fixes this by adding the missing error code `ENOTDIR`.