More informative error messages when adding a federated participant fails

[kaffeeringe]

I tired to open a chat with some external users. Sometimes that fails and the only, generic error is:

"Es ist ein Fehler beim Hinzufügen der Teilnehmer aufgetreten."

I tried to find out, why they failed.

1. One instance restricted federated chats to certain domains. It worked, when we disabled that feature. So it works as intended. But it would be nice if Nextcloud instances were a bit more informative. It should say, that you are not allowed to add this user.
2. One instance timed out.
3. One instance was in maintainance mode.

The error message should say something like "Couldn't reach the other nextcloud. Make sure it is up an running." And then link to the cloud.

Nextcloud 29.0.2