Bug 29646: Fix undefined category crash in opac-password-recovery

Bug 29646: Fix undefined category crash in opac-password-recovery

The crash is a result of a not found borrower. This is typically
a bad or repeated recovery attempt.

Test plan:
Do a password recovery.
Use the mailed URL twice.
Without this patch, the second attempt crashes.
With this patch, the second attempt shows an error dialog.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>