Summary: Fix exception handling in security_group.py

Detail: Code was attempting to concatinate an exception to a string.
Updated to convert to text and concatinate that.

BUG-ID: CLOUDSTACK-1052
Bugfix-for: master
Reported-by: Noa Resare
Signed-off-by: John Kinsella <jlk@stratosec.co> 1363218769 -0700