More defensive code around dedup cache

[rozbb]

This PR does three main things:

1. Discriminates between key-not-found errors and proper errors when fetching head/tail values in DedupCache
2. Checks for more kinds of anomalous behavior of the head/tail, and corrects it
3. Adds some logging and improves existing logging

This also reverts the panic shim commit because it wasn't compiling for me.

Let me know if you'd like more logging, or if this should cover it