Custom exception needs str representation
Custom exception needs str representation

Raising an unhandled OpenStackCloudException should produce output
with the error message. This doesn't happen unless we have a string
representation for our custom exception.

Change-Id: I6b7eab28ffa9385eea9233b0053ade5e3396c9c9

jsmartin/shade,dtroyer/python-openstacksdk,openstack/python-openstacksdk,jsmartin/shade,stackforge/python-openstacksdk,openstack-infra/shade,openstack-infra/shade,openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk