Don't attempt to return users to non-existant transfer

When a registration transfer invoice is completed, if they payment
confirmation notification hits before the user clicks onwards, we can't
find the transfer record because it's been removed. In this case, just
redirect the user back to the invoice so they get something other than a
crash.

postgresqleu/confreg/invoicehandler.py

@@ -333,7 +333,10 @@ def get_return_url(self, invoice):
         try:
             pending = RegistrationTransferPending.objects.get(pk=invoice.processorid)
         except RegistrationTransferPending.DoesNotExist:
-            raise Exception("Could not find pending transfer: %s!" % invoice.processorid)
+            # We can't find the transfer - that could be because it's successfully completed,
+            # at which point the reservation may be canceled. So in this case, we just redirect
+            # the user back to their invoice, because we have to send them somewhere.
+            return "{}/invoices/{}/".format(settings.SITEBASE, invoice.recipient_secret)
 
         return "{}/events/{}/".format(settings.SITEBASE, pending.conference.urlname)