Don't raise exceptions on graceless close cases

[tomchristie]

When a connection fails to close gracefully we're currently raising exceptions.
Actually I really don't think we want to be doing that.

An exception raised during connection close means that the connection wasn't terminated gracefully, but it will still be terminated. Raising exceptions here just means that:

• We can have cases where a request/response cycle runs to completion, but the client library will raise an exception because an unclean close occurred.
• We can have cases where an exception occurs on issuing a request against the connection pool, where we tear down an expired keep alive connection before issuing a fresh request, but an exception ends up being raised.
• We're closing a client and an exception is raised, that we really don't want to occur.