More clear reporting after build job times out

[casparvl]

In this PR the bot reports back 'UNKNOWN' instead of SUCCESS or FAILED. It was explained to me that this really is a timeout of the job that got submitted. It would be nice if we can present a more informative message in this case.

I believe the job output file should contain some message from SLURM stating that the job or job step is cancelled, so we could probably grab for that and if present, report back as a timeout.

[trz42]

It actually could be 'UNKNOWN' for different reasons. At the end of a job, the script bot/check-build.sh provided by the target repository/PR is run. That script will create a file _bot_JOBID.result in the job's working directory. The 'UNKNOWN' status is shown when there is no such file (e.g., job crashed, job was terminated, something else prevented running that script, ...) or if the script could not determine if the job succeeded or failed.