Missing files in Heritrix job directory lead to frozen target instance

[hannakoppelaar]

If a target instance is stopped while one of the H3 job subdirectories "warcs", "reports" and "logs" is missing, a null pointer exception occurs, causing the completeHarvest() method to be rescheduled, leading to an infinite loop. This means the target instance remains stuck in the Stopping state.

We will need to refactor the bookkeeping in this part of the codebase to fix this properly. The impact of this issue is not too big though.