Resultify get_chat_id_by_grpid and create_or_lookup_mailinglist

Use `Option` instead of `Error` to indicate that no chat ID is found.

src/imap.rs

@@ -1636,7 +1636,7 @@ pub(crate) async fn prefetch_should_download(
     // deleted from the database or has not arrived yet.
     if let Some(rfc724_mid) = headers.get_header_value(HeaderDef::MessageId) {
         if let Some(group_id) = dc_extract_grpid_from_rfc724_mid(&rfc724_mid) {
-            if let Ok((_chat_id, _, _)) = get_chat_id_by_grpid(context, group_id).await {
+            if get_chat_id_by_grpid(context, group_id).await?.is_some() {
                 return Ok(true);
             }
         }