[results] write completed.json and error.json to signal instance run success

[phisad]

It might be nice to see directly from the instances dir if the run went through.

Currently we have a hint with a requests.json being written or not.

However, it would be nice to have dedicated files that signal if the instance was only run partially or complete.

[phisad]

This naturally connects to issue #231 to identify if instances should be done again, e.g., the user might want to remove a completed.json to have individual instances to be run again.