Clear previous connection errors

- Related to #37,

Since network connectivity errors are typically transient, consider clearing them for episodes when the network is back into a known-good state.

### Expected behavior

Network connectivity error is cleared for episodes showing it.

### Actual behavior

Network connectivity error persists until the user explicitly retries on a per-episode basis.

### Steps to reproduce the behavior

1. Go into 100% loss mode using a tool such as [network link conditioner](https://nshipster.com/network-link-conditioner/)
2. Attempt to play an episode
3. Wait for the connectivity error to be shown (e.g. "Check your Internet connection and try again.")
4. Observe that the error state is retained between app restarts for all episodes that show it
5. Observe that retrying playback with a known-good network state does not affect any other episodes showing the error

Start by reproducing the issue with Network Link Conditioner, then trace how the playback connectivity error is retained across app restarts and episodes. Done means that episodes showing the error clear it when connectivity returns, without requiring separate retries for each episode.

Written by the indexing model from the issue text.