Bug 17296: Display warning if AnonymousPatron is not correctly set

Bug 17296: Display warning if AnonymousPatron is not correctly set

Bug 14655 added a warning to the about page ("System information" tab)
if the AnonymousPatron feature is not correctly configured.
But actually there is one case when it's not displayed.

Test plan:
Set AnonymousPatron to a non existing patron
Set at least 1 borrowers.privacy == 2
go on the about page.
Without this patch you do not get the warning
With this patch you will see "Some patrons have requested a privacy on
returning item but the AnonymousPatron pref is not set correctly. Set it
to a valid borrower number if you want that this feature works
correctly."

Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 1a2d804a8094ea05199803593698b4294f029913)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit 92d648f579fb03e1e5bed45e28c8604a2ea7aab0)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>