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.
Merge pull request #385 from kri5/wip-1779

rgw: Fix return value for swift user not found

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>