zopewsgi: enable HTTPExceptionHandler

This middleware is enabled in Zope's mkwsginstance [1] and also in test,
[2], we should also enable it here for consistency.

It is necessary to handle exceptions when no standard_error_message
skin exists, like it is the case today in RJS skin selection.

1: https://github.com/zopefoundation/Zope/blob/828b1973e39f4dc1bf51fea0cd11a236d28ec3b5/src/Zope2/utilities/skel/etc/zope.ini.in#L16
2: https://github.com/zopefoundation/Zope/blob/828b1973e39f4dc1bf51fea0cd11a236d28ec3b5/src/Testing/ZopeTestCase/functional.py#L136
13 jobs for erp5-vifib-zope4py2 in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Zope4py2

02:03:38

failed SlapOS-Master.UnitTest-Zope4py2

01:55:40

failed SlapOS-Master.UnitTest-Zope4py2

03:20:34

failed SlapOS-Master.UnitTest-Zope4py2

01:40:06

canceled SlapOS-Master.UnitTest-Zope4py2

failed SlapOS-Master.UnitTest-Zope4py2

02:29:03

failed SlapOS-Master.UnitTest-Zope4py2

01:21:23

failed SlapOS-Master.UnitTest-Zope4py2

06:01:38

failed SlapOS-Master.UnitTest-Zope4py2

01:36:36

failed SlapOS-Master.UnitTest-Zope4py2

02:05:09

failed SlapOS-Master.UnitTest-Zope4py2

01:19:10

failed SlapOS-Master.UnitTest-Zope4py2

01:16:11

failed SlapOS-Master.UnitTest-Zope4py2

01:55:55