Improve failure when a database access happens from unittest.TestCase

[blueyed]

From #197 (comment)

Regarding the error message about the marker, we could probably improve the failure when a database access happens from a unittest.TestCase subclass to instruct the user to subclass from django.test.TestCase instead.

Should this then also happen for access through setUpClass, or should this behavior be left unchanged?

[blueyed]

Assuming this to be fixed/addressed.