connectivity: provide server unavailable error

connectivity: provide server unavailable error

provide server unavailable error(503) value in ocstack

Change-Id: Ica4956debfd8112f93beab4edd23a569ea5a3ad0
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18679
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
(cherry picked from commit 1dcb85b9796760b7e18dfd62ca85614d4d784e40)
Reviewed-on: https://gerrit.iotivity.org/gerrit/21139