Return 404 when AJAX tries to list dir content but file given

Due to a code mistake, the expected 404 return when AJAX tries to list a directory content with a non-directory file path given, does not happen. It instead fails with another exception.

This commit restores the original intention to return 404 in the first place when passing a non-directory path with the "dir" parameter.

Signed-off-by: MichaIng <micha@dietpi.com>