Server-side grouped notifications not working on some servers

### Problem I have

Hi,

First of all, thanks a lot for Phanpy! It really moves the needle in terms of UX and UI; in many ways it's more reactive than native apps. Kudos.

I enabled the server-side grouped notifications (alpha-stage feature, I know), then forgot about it. Later, notifications were not working, and I did not realize both were linked, because the error message just said that fetching notifications failed.

That's because my server (using an old mastodon version / glitch-soc) does not yet support the new notifications API.

### Solution I'd like

It might be worth enhancing this error message ("your server does not support grouped notifications or the v2 api") as this feature eventually moves to beta-stage.

Please consider this as a general UX feedback and how an user can paint themselves in a corner. Feel free to close this issue!

### Alternatives considered

_No response_

### Other

_No response_

Start by tracing the grouped-notifications request and the existing generic notification-fetch failure handling. The issue is done when servers that lack the newer notifications API produce an explanatory error instead of only saying that fetching notifications failed.

Written by the indexing model from the issue text.