ldap: do not crash on timeout in get_ldap_group_dns (#54405)

src/authentic2/backends/ldap_backend.py

@@ -1008,6 +1008,8 @@ class LDAPBackend(object):
                 results = self.normalize_ldap_results(results)
             except ldap.NO_SUCH_OBJECT:
                 pass
+            except ldap.TIMEOUT:
+                log.error('connection timed out while retrieving group DNs')
             else:
                 group_dns.update(dn for dn, attrs in results)
         return group_dns