zfscmd: avoid word "error" in info-level log lines (#873)

A number of tools including `lnav` do a crude string matching against
the word "error". For zrepl this results in benign lines being colored
red. It would be better if the logged message was something like
"command ran successfully".

refs
- #872

---------

Co-authored-by: Christian Schwarz <me@cschwarz.com>

internal/zfs/zfscmd/zfscmd_logging.go

@@ -35,7 +35,7 @@ func waitPostLogging(c *Cmd, u usage, err error, now time.Time) {
 		WithField("usertime_s", u.user_secs)
 
 	if err == nil {
-		log.Info("command exited without error")
+		log.Info("command exited with success")
 	} else {
 		log.WithError(err).Info("command exited with error")
 	}