d/shuffle-binaries: Make error detection/message overt

d/shuffle-binaries: Make error detection/message overt

The reason for the `ls` at the end is pretty straightforward if you think
about it, mainly confirming the shuffle step did something.  Yet that
is a bit non-obvious, and the error message produced on failure is poor
("No such file or directory" simply means the script failed somewhere?).
Add an overt error message.

Signed-off-by: Elliott Mitchell <ehem+debian@m5p.com>
Acked-by: Hans van Kranenburg <hans@knorrie.org>