feat: Don't ignore receive_imf_inner() errors, try adding partially downloaded message instead (#7196)

Ignoring `receive_imf_inner()` errors, i.e. silently skipping messages on failures, leads to bugs
never fixed. As for temporary I/O errors, ignoring them leads to lost messages, in this case it's
better to bubble up the error and get the IMAP loop stuck. However if there's some logic error, it's
better to show it to the user so that it's more likely reported, and continue receiving messages. To
distinguish these cases, on error, try adding the message as partially downloaded with the error set
to `msgs.error`, this way the user also can retry downloading the message to finally see it if the
problem is fixed.

src/download.rs

@@ -238,14 +238,20 @@ impl MimeMessage {
     /// the mime-structure itself is not available.
     ///
     /// The placeholder part currently contains a text with size and availability of the message;
+    /// `error` is set as the part error;
     /// in the future, we may do more advanced things as previews here.
     pub(crate) async fn create_stub_from_partial_download(
         &mut self,
         context: &Context,
         org_bytes: u32,
+        error: Option<String>,
     ) -> Result<()> {
+        let prefix = match error {
+            None => "",
+            Some(_) => "[❗] ",
+        };
         let mut text = format!(
-            "[{}]",
+            "{prefix}[{}]",
             stock_str::partial_download_msg_body(context, org_bytes).await
         );
         if let Some(delete_server_after) = context.get_config_delete_server_after().await? {
@@ -259,9 +265,10 @@ impl MimeMessage {
 
         info!(context, "Partial download: {}", text);
 
-        self.parts.push(Part {
+        self.do_add_single_part(Part {
             typ: Viewtype::Text,
             msg: text,
+            error,
             ..Default::default()
         });