factory: raise on closed connections

When connection is closed by the server, but there is no other error, it
will be close (is_cloes == True) without setting `last_error`. This is
true for all reactors apart from Twisted as far as I can tell.

If we try to use such connection, we'll quickly discover that its
broken, but we can slightly optimize this process by raising directly
from factory().

Author: Lorak-mmk

cassandra/connection.py

@@ -986,6 +986,8 @@ def factory(cls, endpoint, timeout, host_conn = None, *args, **kwargs):
             conn.close()
             raise OperationTimedOut("Timed out creating connection (%s seconds)" % timeout,
                                     timeout=timeout)
+        elif conn.is_closed:
+            raise ConnectionShutdown("Connection to %s was closed by server" % conn.endpoint)
         else:
             return conn