guestfish.js: Fix reference to mntdir in cleanup path
guestfish.js: Fix reference to mntdir in cleanup path

Exception handlers in dynamic languages don't get tested very much =/

chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,chergert/gnome-continuous