test: improve repo_update test fatal message

Signed-off-by: lemonli <liwenjun0323@gmail.com>

cmd/helm/repo_update_test.go

@@ -67,7 +67,7 @@ func TestUpdateCustomCacheCmd(t *testing.T) {
 		t.Fatal(err)
 	}
 	if _, err := os.Stat(filepath.Join(cachePath, "charts-index.yaml")); err != nil {
-		t.Fatalf("error finding created index file in custom cache: %#v", err)
+		t.Fatalf("error finding created index file in custom cache: %#v \ngot: %s", err, out.String())
 	}
 }