Drop a claimed task whose stored payload cannot be read

[codingjoe]

The reaper read the stored payload before deciding the fate of a task, so a payload the code can no longer read, a task function or retry callback that is gone, or a status a newer release wrote, raised on every pass. The claim script had already renewed the lease, so the task stayed in the running set and the same error repeated every CLAIM_TTL, and the inspector could not clear it.

• Drop a claimed task the broker cannot read: log the read error with its traceback, and delete the stored payload.
• Delete only the payload, not the running entry. A lease holder's late acknowledge still writes its result, and the reaper script sweeps the entry once the hash is gone.
• Replace the import-error salvage from Fail tasks whose stored retry callback is gone from the code base #87, which patched and published the payload, with that one drop. The source file is a net deletion.
• Document the behavior in the README.

Fixes #73