Bug 16996: (follow-up) Do not explode if mandatory fields are missing

Bug 16996: (follow-up) Do not explode if mandatory fields are missing

When registering a new patron, if something went wrong, the form is
resent to the template but without the categorycode (not a dropdown list
anymore, but just displayed as a readonly value # TODO LATER).

Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 6cd1891ffe5903a09e2deb04e852e967a2187ba0)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>