Fix run and job status_message calculation

[r4victor]

This PR:

• Fixes run.status_message to be calculated correctly without accessing all the job submissions (job_submission can be limited after Introduce job_submissions_limit for /api/runs/list  #2883).
• Makes run.status_message and job.status_message regular calculable fields instead of validators/computed_fields so that they can be computed effectively before serialization and avoid dict() override tricks.
• Fixed run.status_message calculation in multi-job cases (previously accounted for job[0] only) and worked weirdly with multiple jobs.

The status_message remained the same for most cases.