race condition on terminating job_cmd and error output

[robnagler]

If the job_cmd has an exception, it sends it back as a json string (return from default_command). job_agent is watching for termination, stdout, and stderr. If the job terminates, on_stdout_read doesn't write anything. This also happens if send_reply is false. It's unclear that an exception should be caught at this level. It seems like a pkdlog would be useful in any event.