rgw: Fix return value for swift user not found

http://tracker.ceph.com/issues/1779 fixes #1779

Adjust the return value from rgw_get_user_info_by_swift call
in RGW_SWIFT_Auth_Get::execute() to have the correct
return code in response.
(cherry picked from commit 4089001de1f22d6acd0b9f09996b71c716235551)