tap-bsd: Convert tap_open() to Error

Fixes inappropriate use of stderr in monitor command handler.

While there, improve some of the messages a bit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-13-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>