Grader service should provide grading logs if grading fails before or after _run

[meffmadd]

Is your feature request related to a problem? Please describe.
Right now the grading_logs variable in the graders is always None before the _run method where the executor runs actual the auto-grading job.

Describe the solution you'd like
There should be try statements around _pull_submission etc. in the start method. When catching an error the logs should be set appropriately.