Recognise authentication failures (a RabbitMQ extension)

RabbitMQ >= 3.2.0 sends a connection.close on authentication failure,
rather than dropping the connection, provided the capability
'authentication_failure_close' is flagged by the client.

While I was here, I improved the close messages by giving the constant
name corresponding to the `replyCode` given in the close frame; e.g.,
ACCESS-REFUSED. (As it happens, RabbitMQ includes these in the frame's
`replyText`. Still.)