m/p/supervisor: provide more concise logs on runnable exit

Change-Id: Iba99bdf5d516e0edad6da40951d61894acf0b70b
Reviewed-on: https://review.monogon.dev/c/monogon/+/1357
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/metropolis/pkg/supervisor/supervisor_processor.go b/metropolis/pkg/supervisor/supervisor_processor.go
index ea8a4d0..8b5b336 100644
--- a/metropolis/pkg/supervisor/supervisor_processor.go
+++ b/metropolis/pkg/supervisor/supervisor_processor.go
@@ -297,12 +297,10 @@
 	err := r.err
 	// A lack of returned error is also an error.
 	if err == nil {
-		err = fmt.Errorf("returned when %s", n.state)
-	} else {
-		err = fmt.Errorf("returned error when %s: %w", n.state, err)
+		err = fmt.Errorf("returned nil when %s", n.state)
 	}
 
-	s.ilogger.Errorf("Runnable %s died: %v", n.dn(), err)
+	s.ilogger.Errorf("%s: %v", n.dn(), err)
 	// Mark as dead.
 	n.state = nodeStateDead