mkdfs: improve error message on empty top-level directory
mkdfs: improve error message on empty top-level directory

Currently, mkdfs fails when an empty directory is specified. The error
message is a bit generic and doesn't help the user understanding the
problem. Improve it.

DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon,sp1187/libdragon,sp1187/libdragon,DragonMinded/libdragon