Fix validip6addr bug on Windows

On installing web.py afresh from git on Windows 2.7.2 the tutorial fails with:

```
U:\code\usage\usage> python code.py 3333
Traceback (most recent call last):
  File "code.py", line 13, in <module>
    app.run()
  File "C:\Python27\lib\site-packages\web\application.py", line 313, in run
    return wsgi.runwsgi(self.wsgifunc(*middleware))
  File "C:\Python27\lib\site-packages\web\wsgi.py", line 55, in runwsgi
    server_addr = validip(listget(sys.argv, 1, ''))
  File "C:\Python27\lib\site-packages\web\net.py", line 108, in validip
    if validip6addr(ip): return (ip,port)
  File "C:\Python27\lib\site-packages\web\net.py", line 33, in validip6addr
    socket.inet_pton(socket.AF_INET6, address)
AttributeError: 'module' object has no attribute 'inet_pton'
```

The error is that web/net.py is missing the correct exception at line 34. This PR fixes that.