Fix inconsistent error message when deleting groups that don't exist.

Ticket #292

ipalib/plugins/group.py

@@ -165,7 +165,9 @@ class group_del(LDAPDelete):
         def_primary_group_dn = group_dn = self.obj.get_dn(def_primary_group)
         if dn == def_primary_group_dn:
             raise errors.DefaultGroup()
-        (group_dn, group_attrs) = ldap.get_entry(dn)
+        (group_dn, group_attrs) = self.obj.methods.show(
+            self.obj.get_primary_key_from_dn(dn)
+        )
         if 'mepmanagedby' in group_attrs:
             raise errors.ManagedGroupError()
         return dn