Fix intermittent test failures in OmeroPy-integration-stable.

This PR updates the exceptions expected in ``OmeroPy/test/integration/test_tickets3000.py``. This way ``test2628`` should pass all the time now.

To test this PR, verify that the exception seen in http://hudson.openmicroscopy.org.uk/view/2.%20Stable/job/OmeroPy-integration-stable/67/testReport/junit/test.integration.test_tickets3000/TestTickets3000/test2628/ doesn't happen again for a few next builds.