Bug 19760: Die if koha-conf is not accessible

Bug 19760: Die if koha-conf is not accessible

We got a lot of support questions because scripts are executed without
the correct environment/user.
We could die and provide more useful information

Tested on my kohadevbox VM by:

1) sudo chmod a-rx /etc/koha/sites/kohadev/koha-conf.xml
2) run help.pl from the command line to produce the desired die message
3) sudo chmod a+rx /etc/koha/sites/kohadev/koha-conf.xml
4) rerun help.pl successfully

Signed-off-by: Jon Knight <J.P.Knight@lboro.ac.uk>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Bug 19760: Change wording

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>