Do not log unexpected process kills to STDOUT
Do not log unexpected process kills to STDOUT

There's a race condition in SlaveNode in which we see a stale child
process boot and need to kill it:
* Send a request to boot a child process.
* File change causes us to kill the parent.
* Child boots but is associated with an old parent so we kill it.

This is harmless but currently logs a scary error to STDOUT. We should send
this to the trace log instead.

burke/zeus,burke/zeus,burke/zeus