Smaller improvements to TCP side of new akka-io implementation

Main thing:

Change the TcpOutgoingConnection to respond with a `Tcp.CommandFailed` reply rather than `Tcp.ErrorClosed` on failed connects. For symmetry with the `Tcp.Bind` case and general usability of the API a `Tcp.CommandFailed` is the better choice. (As an API user I have to watch for the `Tcp.CommandFailed` anyway since it's going to be coming back if the selector is at max capacity.)