Fix error message when isolated tenant subnet creation fails

The current code assumes that setting the message of an exception will
cause the desired message to be displayed but this does not appear to be true.

Change-Id: I7e91179e7cb426645e9250671d3b54475f79a613
Merge "Fix error message when isolated tenant subnet creation fails"
Merge branch 'master' of https://github.com/openstack/tempest