DV2 DetectionBase knows more about allocation statuses

This PR was spawned from comments in #345.  It contains a chunk of refactoring, followed by a few feature adds:

- give a specific error message when an allocation is archived
- notice purged or invalid allocation links and continue seamlessly
- add a bit more instrumentation
I agree that this is fishy and we probably do want to leave the sym link there if we're running on the same `test_name`.