feat(crons): Update processing errors storage to store only identifiers in the set

[wedamija]

We need to support deleting processing errors when a user dismisses them, and the current data storage format doesn't support that well. This modifies the format to store only the guid in the set, so we can easily look it up and delete it. The full error is stored in a separate key.

There's a chance that the set might contain errors that have ttled out - we don't care if this happens, these are too old to be actionable, so we just skip rows with no data.