[1.6.x] Fixed "Address already in use" from liveserver.

Our WSGIServer rewrapped the socket errors from server_bind into
WSGIServerExceptions, which is used later on to provide nicer
error messages in runserver and used by the liveserver to see if
the port is already in use. But wrapping server_bind isn't enough since
it only binds to the socket, socket.listen (which is called from
server_activate) could also raise "Address already in use".

Instead of overriding server_activate too I chose to just catch socket
errors, which seems to make more sense anyways and should be more robust
against changes in wsgiref.

Backport of 2ca00faa91 from master.

tests/servers/tests.py

@@ -5,10 +5,10 @@ Tests for django.core.servers.
 from __future__ import unicode_literals
 
 import os
+import socket
 
 from django.core.exceptions import ImproperlyConfigured
 from django.test import LiveServerTestCase
-from django.core.servers.basehttp import WSGIServerException
 from django.test.utils import override_settings
 from django.utils.http import urlencode
 from django.utils.six.moves.urllib.error import HTTPError
@@ -71,7 +71,7 @@ class LiveServerAddress(LiveServerBase):
         cls.raises_exception('localhost', ImproperlyConfigured)
 
         # The host must be valid
-        cls.raises_exception('blahblahblah:8081', WSGIServerException)
+        cls.raises_exception('blahblahblah:8081', socket.error)
 
         # The list of ports must be in a valid format
         cls.raises_exception('localhost:8081,', ImproperlyConfigured)