fix: return correct exit code on SIGTERM (#1530)

SIGTERM (15) is 128+15=143 (not 137).

See https://tldp.org/LDP/abs/html/exitcodes.html and
https://komodor.com/learn/exit-codes-in-containers-and-kubernetes-the-complete-guide/
for details.

Fixes #1529.

cmd/errors.go

@@ -26,7 +26,7 @@ var (
 
 	errSigTerm = &exitError{
 		Err:  errors.New("SIGTERM signal received"),
-		Code: 137,
+		Code: 143,
 	}
 )