slapos_payzen: Ensure transaction is not None before process

    This also changes the raise for a more appropriated Exception

master/bt5/slapos_payzen/SkinTemplateItem/portal_skins/slapos_payzen/PayzenEvent_processUpdate.py

@@ -10,6 +10,9 @@ def storeWorkflowComment(ctx, comment):
 payzen_event = context
 transaction = payzen_event.getDestinationValue()
 
+if transaction is None:
+  raise ValueError("Unable to find related transaction")
+
 assert signature in (True, False)
 if signature is False:
   # signature is wrong, bye bye

master/bt5/slapos_payzen/TestTemplateItem/portal_components/test.erp5.testSlapOSPayzenSkins.py

@@ -189,7 +189,7 @@ class TestSlapOSPayzenEvent_processUpdate(SlapOSTestCaseMixinWithAbort):
   def test_processUpdate_noTransaction(self):
     event = self.createPayzenEvent()
     self.assertRaises(
-      AttributeError,
+      ValueError,
       event.PayzenEvent_processUpdate,
       'a', 'b')