Correct 4xx/5xx response management in SessionClient

Currently SessionClient raises an exception when status code >= 400
unlike HTTPClient. neutronclient.v2_0.client expects HTTPClient
behavior.

This change disables exception raise when status code >= 400 in
SessionClient (more precisely in keystoneclient.session).

Change-Id: Ic96cb9326842f0b6a180fd8f94293da9fa0f6b2f
Closes-Bug: #1376171
Merge "Correct 4xx/5xx response management in SessionClient"