Allow ponos state transition from pending to failed & completed

Refs https://gitlab.teklia.com/arkindex/ponos-agent/-/merge_requests/82 & https://redmine.teklia.com/issues/6908

In some cases, the ponos (slurm) agent can trigger a task, and the task fails or completes before the next iteration.

When a task is triggered in slurm it's still pending, as the slurm manager decides itself when it will run (so it's not the exact same workflow as docker ponos)

This means a task can become completed or failed, coming from a pending state.

Please allow these transitions (bonus point if it's only for a slurm agent)