Replay a unanimous upstream rejection instead of a 502

Every backend gets the same query, so one they all refuse is the client's
mistake; flattening it into "all backends failed" threw openvoxdb's own
explanation away and logged a typo as an outage.

- Carry status, content type and body on a typed upstreamError
- Replay the status and explanation when every backend refuses alike
- Redact backend addresses from replayed bodies
- Keep a refused query out of the partial counters and the cache

server_test.go

@@ -31,6 +31,10 @@ type fakeBackend struct {
 	totals map[string]int
 	fail   bool          // return 500 for everything
 	delay  time.Duration // artificial latency
+	// reject answers every request with this status and rejectBody, standing in
+	// for a PuppetDB refusing a query it cannot answer.
+	reject     int
+	rejectBody string
 
 	mu        sync.Mutex
 	gotParams map[string]url.Values
@@ -60,6 +64,10 @@ func newFakeBackend(t *testing.T, nodesBody, factsBody string) *fakeBackend {
 			http.Error(w, "boom", http.StatusInternalServerError)
 			return
 		}
+		if fb.reject != 0 {
+			http.Error(w, fb.rejectBody, fb.reject)
+			return
+		}
 		if body, ok := fb.bodies[r.URL.Path]; ok {
 			if n, ok := fb.totals[r.URL.Path]; ok && r.URL.Query().Get("include_total") == "true" {
 				w.Header().Set(recordsHeader, strconv.Itoa(n))