Return sink write errors from every handler (closes #22)

The console and JSON handlers threw away the error from their write to
stdout, so a lost log line looked delivered. They now return it,
wrapped. The webhook handler also returns an error for a status outside
2xx. The multiplex handler passes the record to every handler and
returns their errors joined with errors.Join instead of stopping at the
first.

Both stdout handlers gain an unexported writer, nil meaning os.Stdout at
write time, so tests can supply a sink that fails. The README says what
each handler returns and that the slog.Logger methods discard a
handler's error.

Model: opus-5-5

README.md

@@ -125,7 +125,8 @@ error away. That is how `log/slog` works, and simplelog cannot change it.
 To find out whether a record was delivered, build a `slog.Record` and pass
 it to the handler yourself, for example
 `slog.Default().Handler().Handle(ctx, record)`, then check the error it
-returns.
+returns. A record passed to `Handle` directly gets the wrong file and
+line in console output.
 
 ## Entrypoints
 

handler_errors_internal_test.go

@@ -82,6 +82,38 @@ func TestMultiplexHandlerDeliversPastFailingHandler(t *testing.T) {
 	}
 }
 
+// When more than one handler fails, the caller gets every failure, not
+// only the first.
+func TestMultiplexHandlerReturnsEveryFailure(t *testing.T) {
+	t.Parallel()
+
+	server := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusInternalServerError)
+		},
+	))
+	defer server.Close()
+
+	webhook, err := NewWebhookHandler(server.URL)
+	if err != nil {
+		t.Fatalf("NewWebhookHandler: %v", err)
+	}
+
+	handler := &MultiplexHandler{handlers: []ExtendedHandler{
+		&JSONHandler{out: failingWriter{}},
+		webhook,
+	}}
+
+	err = handler.Handle(context.Background(), errorTestRecord())
+	if !errors.Is(err, errSinkFailed) {
+		t.Fatalf("Handle returned %v, want the JSON handler's error", err)
+	}
+
+	if !errors.Is(err, errWebhookStatus) {
+		t.Fatalf("Handle returned %v, want the webhook's error", err)
+	}
+}
+
 func TestWebhookHandlerReturnsErrorOnServerError(t *testing.T) {
 	t.Parallel()