flux-imp kill: stderr: flux-imp: Fatal: kill: failed to initialize pid info: Invalid argument

[grondo]

I think this error occurs when there are no IMP or job shell processes left so the target PID can't be found (have to verify this). The error is obtuse however, and the IMP should just avoid printing anything if the target PID isn't found, as this could be an expected condition if there's a race between the kill and an already exiting process.