Do not warn about lease refreshes on destroyed machines (#4167)

internal/machine/leasable_machine.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 	"sync"
 	"time"
 
@@ -498,23 +499,25 @@ func (lm *leasableMachine) StartBackgroundLeaseRefresh(ctx context.Context, leas
 }
 
 func (lm *leasableMachine) refreshLeaseUntilCanceled(ctx context.Context, duration time.Duration, delayBetween time.Duration) {
-	var (
-		err error
-		b   = &backoff.Backoff{
-			Min:    delayBetween - 20*time.Millisecond,
-			Max:    delayBetween + 20*time.Millisecond,
-			Jitter: true,
-		}
-	)
+	b := &backoff.Backoff{
+		Min:    delayBetween - 20*time.Millisecond,
+		Max:    delayBetween + 20*time.Millisecond,
+		Jitter: true,
+	}
+
 	for {
-		err = lm.RefreshLease(ctx, duration)
-		switch {
+		time.Sleep(b.Duration())
+		switch err := lm.RefreshLease(ctx, duration); {
+		case err == nil:
+			// good times
 		case errors.Is(err, context.Canceled):
 			return
-		case err != nil:
+		case strings.Contains(err.Error(), "machine not found"):
+			// machine is gone, no need to refresh its lease
+			return
+		default:
 			terminal.Warnf("error refreshing lease for machine %s: %v\n", lm.machine.ID, err)
 		}
-		time.Sleep(b.Duration())
 	}
 }