Clarify error messages in HPA metrics

With the introduction of the RESTMetrics client, there are two ways to
fetch metrics for auto-scaling. However, they previously shared error
messages. This could be misleading. Make the error message more clearly
show which method is in use.

pkg/controller/podautoscaler/metrics/rest_metrics_client_test.go

@@ -252,7 +252,7 @@ func TestRESTClientQpsSumEqualZero(t *testing.T) {
 func TestRESTClientCPUEmptyMetrics(t *testing.T) {
 	tc := restClientTestCase{
 		resourceName:         v1.ResourceCPU,
-		desiredError:         fmt.Errorf("no metrics returned from heapster"),
+		desiredError:         fmt.Errorf("no metrics returned from resource metrics API"),
 		reportedMetricPoints: []metricPoint{},
 		reportedPodMetrics:   [][]int64{},
 	}