wsgiref compatibility fixes

I was getting pywb running alongside Django (which uses wsgiref for the dev server instead of uwsgi) and had to tweak a couple of things:

- Manually set env[‘REQUEST_URI’] (which is nonstandard) the same way it’s set by uwsgi.
- Include HTTP error code reasons in error response. (wsgiref checks that the response status is at least 4 characters, i.e. includes the text description as well as number).