Delete bad chunks with atticadm

[gmacon]

The FAQ says about missing/corrupt chunks:

The atticadm utility will have the functionality to kill/delete bad chunks.

This is a function I've discovered that I need, and I'm willing to work on an implementation, but I have some questions about the best way to proceed:

• From the description, it seems that we need to identify which chunks are bad, knowing which nars are bad is insufficient. What is the best way to identify that a chunk is bad? I can pretty easily figure out which nars are bad by reading the logs when nix build fails, but it's not obvious to me how to figure out which chunk is bad.
• Would it make sense to expose this through the API, potentially with a new claim to allow it to happen?