If Zope is run as a WSGI app, this avoids doing any ZServer related work.

This avoids various import dependencies between the Startup code
and ZServer/webdav.

Based on https://gist.github.com/optilude/1548061.

This serves the same purpose as `egg:paste#httpexceptions`, but
recognizes exceptions derived from `zExceptions.HTTPException`.

This avoids the `AssertionError: app must call start_response before returning`
error message.

Currently this is not fully functional. repoze.retry causes errors
for failing to call start_response. Exceptions like NotFound aren't
turned into their HTML form by neither WebError nor paste#httpexceptions.

This is a very limited copy of Pyramid's pserve script.