Guard a StatusUnprocessableEntity with check for HTMx request

Otherwise, HTMx would just ignore the HTML returned by the server and
dispatch an error.

pkg/products.go

@@ -137,7 +137,9 @@ func HandleUpdateProduct(w http.ResponseWriter, r *http.Request, params httprout
 	}
 	slug := params[0].Value
 	if !form.Validate() {
+		if !IsHTMxRequest(r) {
 			w.WriteHeader(http.StatusUnprocessableEntity)
+		}
 		mustRenderEditProductForm(w, r, slug, form)
 		return
 	}