The original code merely assumed that an XOpenDisplay failure was caused
only by incapacity to connect to the X server, but if $DISPLAY was not set
then the fprintf would be given a NULL pointer which it may not appreciate
(depending on the libc in use).
Let's avoid that case and try to provide more info to help the user.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>