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.

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.

Changes:

- [x] Integration test (should fail without the following changes)
- [x] Check if artifact exists when executing, prepare if not
- [x] Return an internal error when file is missing
- [x] Retry once on internal errors
- [x] Document design (update impl guide)

…-artifact