Have exceptions print something by default
Have exceptions print something by default

Set the message of the base exception so that exceptions convert
to at least some useful string.

Change-Id: Ib5ec54fa27bfc6d7733e1c5726e252b241bcb1f4

briancurtin/python-openstacksdk,dudymas/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,mtougeron/python-openstacksdk,mtougeron/python-openstacksdk,dudymas/python-openstacksdk,openstack/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,briancurtin/python-openstacksdk