heuristics: Weight files with a valid failed thumbnail a lot less
heuristics: Weight files with a valid failed thumbnail a lot less

This should prevent us picking an un-thumbnailable file to represent its
directory.

pwithnall/gnome-directory-thumbnailer,GNOME/gnome-directory-thumbnailer,pwithnall/gnome-directory-thumbnailer,GNOME/gnome-directory-thumbnailer