Remove the deprecated access to Exception message
Remove the deprecated access to Exception message

Deprecated since Python 2.6, removed in Python 3.
Even if APIException from saharaclient provides an error_message
property, this seems the normal way to access the message from
children of Exception.

Change-Id: Icc23553649280bfd8c1c376be2492b08f6388a91

openstack/sahara-scenario,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests