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