[Feature Request]: Add fallback behavior for RSS ratelimiting

### Guidelines

- [x] I have [searched the issue tracker for open and closed issues](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aissue+sort%3Arelevance-desc) that are similar to the feature request I want to file, without success.
- [x] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.
- [x] This issue contains only one feature request.
- [x] I have read and agree to follow the [rules](https://docs.freetubeapp.io/community/rules/).

### Problem Description

When YouTube is having a RSS outage on the RSS feeds users are met with the following on our Subscription page

* Your subscribed channels currently does not have any videos
* Channels with errors

While technically correct it isnt very user friendly.

### Proposed Solution

Show the videos that were listed before refreshing the subscriptions and show a toast that mentions that fetching failed instead of the things listed above

### Alternatives Considered

None.

### Issue Labels

ease of use improvement, display more information to user, visual improvement

### Additional Information

_No response_

Start at the Subscription page and trace how RSS refresh failures currently produce the empty-subscription and channel-error messages. Check how previously listed videos and toast notifications are handled. Done means the prior videos remain visible after an RSS failure and a toast reports that fetching failed.

Written by the indexing model from the issue text.