slapos_payzen: Ensure transaction is not None before process

    This also changes the raise for a more appropriated Exception
9 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:32:37

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:02

failed SlapOS.Eggs.UnitTest-Master.Python3

00:12:48

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:23

passed SlapOS.Eggs.UnitTest-Master.Python2

00:21:24

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:36

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:48

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:40

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:52