Bug 8279: CAS Debugging improvements

Bug 8279: CAS Debugging improvements

  Adds more precise debug informations for easier CAS troubleshootings resolution.

  Before this patch, whenever ticket validation failed, the debug message was "Invalid ticket".
  But ticket validation may fail for other reasons: CAS server not reachable, casServerUrl syspref is wrong...

  This patch adds the reason for ticket validation failing.

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