batch job logs when job is not finished or even started

[soxofaan]

GET /jobs/{job_id}/logs

It's not specified explicitly how back-end should behave when a batch job is still "created"/"queued".
In VITO backend we currently throw JobNotStarted error, because that seemed fitting, but (D28) validator does not like that.
Should we just return empty list of log entries?

[m-mohr]

Logs can be used at all stages of a job, but I see that this part can be a bit misleading:

If requested consecutively while a job is running, it is RECOMMENDED that clients use the offset parameter to get only the entries they have not received yet.

I'll try to improve and issue a PR.

[m-mohr]

see PR #316