bug#1607669: Validate the results of ldap_search() and ldap_list() when syncing users.

When retrieving data from the LDAP server, check the results retrieved from
ldap_search() and ldap_list(). If there is an error, do not continue.

Otherwise, the sync_users() will assume that zero users were retrieved
from LDAP. It will then deleted or suspended users when
'If a user is no longer present in LDAP' is set to either suspend
users or delete them.

behatnotneeded

Change-Id: Ib57901c61f769d96720cf932d2e0d7f643853a56
(cherry picked from commit 1daee33a)