Use str() to generate IOError exception message
Use str() to generate IOError exception message

In python3 IOError has no 'message' property.

Partially-Implements: blueprint magnum-python3

Change-Id: I48f7e1304883357d7d909f742444a4d5fcdc9499

ArchiFleKs/magnum,openstack/magnum,jay-lau/magnum,ArchiFleKs/magnum,openstack/magnum