cls_rbd: do not make noise in osd log on rbd removal

ubuntu@burnupi06:~$ tail -f /var/log/ceph/ceph-osd.1.log
2013-02-07 17:00:30.565749 7fdb09e6b700 0 <cls> cls/rbd/cls_rbd.cc:1615: error reading id for name 'sds': -2
2013-02-07 17:00:30.566301 7fdb0a66c700 0 <cls> cls/rbd/cls_rbd.cc:1521: error reading name to id mapping: -2
2013-02-07 17:03:54.085700 7fdb0a66c700 0 <cls> cls/rbd/cls_rbd.cc:1615: error reading id for name 'sdfsd': -2
2013-02-07 17:03:54.086143 7fdb09e6b700 0 <cls> cls/rbd/cls_rbd.cc:1521: error reading name to id mapping: -2

Fixes: #4047
Signed-off-by: Sage Weil <sage@inktank.com>