[Feature Request]: Show a specific message when a playlist is unviewable

### 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

Currently, the playlist view shows the same message `This playlist currently has no videos` no matter if the playlist has no videos, is private, or does not exist.

### Proposed Solution

Innertube is throwing an error when the playlist is inaccessible/invalid: `InnertubeError: This playlist type is unviewable.`, which allows to differentiate them from empty playlsts.

Instead of `This playlist currently has no videos`, the message could be `This playlist is unviewable`

### Alternatives Considered

Keep `This playlist currently has no videos`,

### Issue Labels

display more information to user

### FreeTube Version

v0.25.1-beta

### Operating System Version

Fedora

### Installation Method

Flathub

### Upload screenshots or videos

_No response_

### Additional Information

_No response_

Start at the playlist view and trace how the Innertube error `This playlist type is unviewable.` is handled. Confirm that inaccessible or invalid playlists receive a distinct message while genuinely empty playlists retain the existing message, then verify the behavior in the app.

Written by the indexing model from the issue text.