Exit with better error message if running with toram=... and invoking with /live...

If running in RAM mode without all the files in /live/image
(happens if using boot option "toram=....squashfs", but NOT when
using just "toram"!) then there are several files missing. We
could use kernel + initrd from /boot, but files like logo.16
aren't available at all. Inform the user about this situation and
possible workarounds.