Show error when model deletion fails

ModelDelete now checks the DB error (e.g. FK constraint from devices still
using the model) and renders the model list with an alert instead of
silently redirecting with no feedback.

internal/handlers/models.go

@@ -146,10 +146,21 @@ func (h *Handlers) ModelUpdate(w http.ResponseWriter, r *http.Request) {
 func (h *Handlers) ModelDelete(w http.ResponseWriter, r *http.Request) {
 	idStr := r.PathValue("id")
 	id, _ := strconv.ParseInt(idStr, 10, 64)
-	h.Store.ModelDelete(id)
+	if err := h.Store.ModelDelete(id); err != nil {
+		h.renderModelListError(w, "Cannot delete model: it is in use by existing devices.")
+		return
+	}
 	http.Redirect(w, r, "/models", http.StatusSeeOther)
 }
 
+func (h *Handlers) renderModelListError(w http.ResponseWriter, msg string) {
+	list, _ := h.Store.ModelGetAll()
+	if list == nil {
+		list = []models.DeviceModel{}
+	}
+	h.render(w, "model_list.html", ModelListData{Models: list, Error: msg})
+}
+
 func saveUploadedFile(r *http.Request, field, dir string) string {
 	file, header, err := r.FormFile(field)
 	if err != nil {