x/crypto/ssh: interpret disconnect message as error in the transport layer.

This ensures that higher level parts (e.g. the client authentication
loop) never have to deal with disconnect messages.

Fixes https://github.com/coreos/fleet/issues/565.

Change-Id: Ie164b6c4b0982c7ed9af6d3bf91697a78a911a20
Reviewed-on: https://go-review.googlesource.com/20801
Reviewed-by: Anton Khramov <anton@endocode.com>
Reviewed-by: Adam Langley <agl@golang.org>