Bug 17095: (bug 16849 follow-up) Fix regression if patron does not exist

Bug 17095: (bug 16849 follow-up) Fix regression if patron does not exist

Bug 16082, 'Display a friendly "patron does not exist" message if that's the
case - circulation.pl' changed the behavior of the checkout page so that if
you hit that page with a borrowernumber which doesn't exist a nice message will
tell you so.

Now when you do that you get an error message:
  Can't call method "is_debarred" on an undefined value at /circ/circulation.pl line 300.

Test plan:
confirm that the friendly message is back :)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>