#4 raise event for connection failed instead of return error in handle_message

[sbernard]

When handle_alert, in handle message line 3655, if peer must be released handle_message return -1. This means handle message failed.
But not sure we must considered a close_notify or a fatal_alert as an handling message error.
Maybe the code is finally a regression.

To allow user to know if connection failed or if a peer is disconnected, we should rather add new events.

[Olaf Bergmann]

The user API is something that needs definitely more work. This includes reasonable notifications of alert messages.