future: catch unknown exceptions in callbacks

Change-Id: I4d09ea6779a2d9a76ec8c572103e65822f4b1079
Reviewed-on: http://gerrit.aldebaran.lan:8080/17516
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: cgestes <gestes@aldebaran-robotics.com>