Replace str() with six.text_type() where exception is used

When switching to oslo.i18n, tests where used str(exc) will failed
with unicodeerror, so before switching to oslo.i18n, need to
replace str with six.text_type where exception is used. And it's
recommanded to use six.text_type instead of str in openstack.

Change-Id: Ie37a6ef74a12b2580ce72afc75c99bcd19d4cf97