When a Worker indicates a task failed, mark it as `soft-failed` until
enough workers have tried & failed at the same task.
This is the first step in a blocklisting system, where tasks of an
often-failing worker will be requeued to be retried by others.
NOTE: currently the failure list of a task is NOT reset whenever it is
requeued! This will be implemented in a future commit, and is tracked in
`FEATURES.md`.