Fix stack traces discovered with ^C forceful termination.

  * utils.py:_kill_process_tree(): Ignore NoSuchProcess errors

    These are caused because we issue SIGTERM, and if the process
    has not exited after a timeout, we kill it.

  * _scheduler/jobs/job.py: Stop handling NoSuchProcess errors here
    redundantly, they are already ignored.

It seems that we were ignoring it after sleeping when terminating
tasks from the scheduler... but we were not ignoring it when performing
the same pattern in the `Plugin.call()` -> `utils._call()` path, so
we were still getting these exceptions at termination time from host
tool processes launched by source plugins.