Bug 16569 - Message box for "too many checked out" is empty if AllowTooManyOverride...

Bug 16569 - Message box for "too many checked out" is empty if AllowTooManyOverride is not enabled

If the limit for number of items checked out is reached, the message box
shows up but is empty.

Test Plan:
1) Disable AllowTooManyOverride
2) Check out items to a patron until the patron has reached the limit
   of checkouts he or she can have
3) Try to check out one more item
4) Note the empty message box
5) Apply this patch
6) Try to check out one more item again
7) Note the message is now visible

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>