Tests for issues in the node->label store consistency checker.

Since the name of the checker class indicates that it would report orphan
records, (failing) tests are added for such scenarios.
Since the checker reads full chains it needs to be able to gracefully handle
records inconsistent in such a way that they form a cyclic chain, a (failing)
test is added for this.
It was discovered that exceptions in the checker were not always propagated,
the code has been refactored to ensure that they are. This revealed further
failures.