Rewrite ensure_dirs_exist in thumbnail_manager
Rewrite ensure_dirs_exist in thumbnail_manager

The previous implementation did not work if $XDG_CACHE_HOME or
$XDG_CACHE_HOME/thumbnails/fail/ were not existent yet as pythons
os.mkdir does not work like mkdir -p.

karlch/vimiv,karlch/vimiv,karlch/vimiv