PVF: Don't dispute on missing artifact

[mrcnski]

ISSUE

Overview

A dispute should never be raised if the local cache doesn't provide a certain artifact. You can not dispute based on this reason, as it is a local hardware issue and not related to the candidate to check.

_Originally posted by @bkchr in paritytech/polkadot-sdk#685

Design

Currently we assume that if we prepared an artifact, it remains there on-disk until we prune it, i.e. we never check again if it's still there.

We can change it so that instead of artifact-not-found triggering a dispute, we retry once (like we do for AmbiguousWorkerDeath, except we don't dispute if it still doesn't work). And when enqueuing an execute job, we check for the artifact on-disk, and start preparation if not found.