Add a missing StatusUnprocessableEntity HTTP response code

pkg/products.go

@@ -124,6 +124,7 @@ func HandleUpdateProduct(w http.ResponseWriter, r *http.Request, params httprout
 		return
 	}
 	if !form.Validate() {
+		w.WriteHeader(http.StatusUnprocessableEntity)
 		mustRenderEditProductForm(w, r, form)
 		return
 	}