Bug 20363: (bug 16966 follow-up) better messages handling

Bug 20363: (bug 16966 follow-up) better messages handling

User views the "Your privacy" -page, it shows "No reading history to
delete", even when the user has history.

Test plan:
- Turn OPACPrivacy on
- Go to opac-privacy.pl
- Click the "Immediate deletion button" to delete the reading history
=> If the patron has reading history you must get "Your reading history has been deleted."
Otherwise "No reading history to delete"

Note that this patch reintroduce the "something went wrong" message if
the deletion failed for whatever reason.

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>