messenger: count the store's writes, and name an ask's record when it is the only one held (review of issue 422)

The review's two nits. A write of bytes identical to what is stored was
invisible to a comparison of content, so the test store counts its
writes and the preview's tests read that count: a persist from inside
Preview now fails five of them instead of one. And a router holding an
ask's record and no condition said it held none, rather than saying what
that key is; the branches are the other way round, with a test that
holds nothing else.

modules/messenger/cmd/messenger/preview.go

@@ -68,11 +68,14 @@ func (h *Holder) Preview(key, what string) (map[string]any, error) {
 	if !holding || r.Origin != "condition" {
 		sort.Strings(conditions)
 		switch {
-		case len(conditions) == 0:
-			return nil, errorf("no condition %s is open here, and this router holds none: `mesh-controller.conditions` says what the controller holds open", key)
+		case holding && len(conditions) == 0:
+			return nil, errorf("%s is not a condition but this router's own %s record, which is said as part of an ask, never on its own; it holds no condition",
+				key, r.Origin)
 		case holding:
 			return nil, errorf("%s is not a condition but this router's own %s record, which is said as part of an ask, never on its own; it holds the conditions %s",
 				key, r.Origin, strings.Join(conditions, ", "))
+		case len(conditions) == 0:
+			return nil, errorf("no condition %s is open here, and this router holds none: `mesh-controller.conditions` says what the controller holds open", key)
 		default:
 			return nil, errorf("no condition %s is open here; it holds %s", key, strings.Join(conditions, ", "))
 		}