fix: do not use square brackets error for unknown sender

If the sender of the message in protected group chat
is not a member of the chat, mark the sender name with `~`
as we do it in non-protected chats and set the error
instead of replacing the whole message with
"Unknown sender for this chat. See 'info' for more details."

To send a message to a protected group this way
the sender needs to know the group ID
and sign the message with the current verified key.
Usually this is just a late message
delivered shortly after the user has left
the group or was removed from it.

Replacing the message with a single error text part
as done before this change makes it impossible
to access anything other than text, such as attached images.

src/stock_str.rs

@@ -149,7 +149,7 @@ pub enum StockMessage {
                     however, of course, if they like, you may point them to 👉 https://get.delta.chat"))]
     WelcomeMessage = 71,
 
-    #[strum(props(fallback = "Unknown sender for this chat. See 'info' for more details."))]
+    #[strum(props(fallback = "Unknown sender for this chat."))]
     UnknownSenderForChat = 72,
 
     #[strum(props(fallback = "Message from %1$s"))]
@@ -930,7 +930,7 @@ pub(crate) async fn welcome_message(context: &Context) -> String {
     translated(context, StockMessage::WelcomeMessage).await
 }
 
-/// Stock string: `Unknown sender for this chat. See 'info' for more details.`.
+/// Stock string: `Unknown sender for this chat.`.
 pub(crate) async fn unknown_sender_for_chat(context: &Context) -> String {
     translated(context, StockMessage::UnknownSenderForChat).await
 }