Monitor: show job success status

[Robert Sachunsky]

We currently only show the termination status by looking at the PID on the Controller (which is communicated via a file $WORKDIR/ocrd.pid):
https://github.com/markusweigelt/ocrd_manager/blob/daed8299411dfb4f3476c5d8ea602ab9ac20c3a4/ocrd_monitor/serve.py#L182-L184

But more importantly, we should also communicate the exit value (of the Manager script, including ocrd process command, resynchronization, ocrd workspace validate command etc).

To differentiate between "total" and "partial" failures, and at what stage, we still need the last lines of the log (persisted job-locally under $WORKDIR/ocrd.log), though.

To achieve that, we could:

• amend logexit and close by persisting $? into (say) $WORKDIR/ocrd.ret
• evaluate that file in the Monitor in case the process has already terminated
• add a link for the local logs in the job view