Bug 8572: invalid auth shouldn't cause error

Bug 8572: invalid auth shouldn't cause error

Prior to this patch, attempting to view an invalid authority in the
OPAC gave a system error instead of a 404. With this patch, the user
will just get a 404, as is the case with invalid bibs.

To test:
1) Try to view an invalid authority by going to:
   /cgi-bin/koha/opac-authoritiesdetail.pl?authid=999999999999
   (presuming that you don't have an authority with authid 999999999999
   in your database) before and after applying this patch

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>