openshift-java-client: dont swallow domain deletion errors in tests if deletion is pre-condition

When running UserResourceIntegrationTests there are currently several tests that fail because of an issue in OpenShift (reported in https://bugzilla.redhat.com/show_bug.cgi?id=916323). The UserResourceIntegrationTests therefore fail in

#shouldCreateDomain
#shouldReturnThatHasNoDomain
#shouldReturnEmptyDomains
#shouldNoDefaultDomainAfterRefresh

These tests rely on the fact that the domain(s) could be deleted, no domain is a pre-condition to them.
Unfortunately these tests swallow the exceptions that may occurr when killing these domains. The tests therefore report resulting errors, not the real cause.
We have to ensure exception that occurr when killing domains are not swallowed. They have to make the test fail (since domain deletion is a precondition).