Do not log unexpected process kills to STDOUT

[metcalf]

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.

r? @ptarjan

[ptarjan]

yay thanks lgtm