Category.vue: improve Error Handling if loading fails

if this fails, the loading state never resolves.
Maybe use a computed of this.category()._meta.loading?
Maybe the error is already catched in the global error handler and shown, else i would catch the error here manually and show a toast

_Originally posted by @BacLuc in https://github.com/ecamp/ecamp3/pull/4362#discussion_r1439062400_