ERP5Type/patches/WSGIPublisher: Tolerate missing PARENTS during error handling.

This can happen if there is an error very early during publication.
9 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:00:19

passed ERP5.PerformanceTest-Master

00:24:17

passed ERP5.UnitTest-Master

04:13:31

failed ERP5.UnitTest-Master.Medusa

03:46:17

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:32

passed SlapOS.Eggs.UnitTest-Master.Python3

00:52:21

failed ERP5.UnitTest-Master

02:23:19

passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:29

passed SlapOS.Eggs.UnitTest-Master.Python3

00:38:04