Connectivity view: Only set smtp to "connected" if the last message was actually sent (#2541)

fix #2539

It's always a bit ambiguous which function should do set_err or set last_send_error, I used this rule here:

If some code returns Status::RetryLater, then it sets last_send_error, because Status::RetryLater means that the user won't see the error directly on the message (if we returned Status::Failed(Err(_)), then the message would be shown as failed to the user) Also, smtp_send always sets last_send_error because, well, sending just failed or succeeded.

Also, remove unused field pending_error.

src/scheduler.rs

@@ -298,7 +298,11 @@ async fn smtp_loop(ctx: Context, started: Sender<()>, smtp_handlers: SmtpConnect
                 None => {
                     // Fake Idle
                     info!(ctx, "smtp fake idle - started");
-                    connection.connectivity.set_connected(&ctx).await;
+                    match &connection.last_send_error {
+                        None => connection.connectivity.set_connected(&ctx).await,
+                        Some(err) => connection.connectivity.set_err(&ctx, err).await,
+                    }
+
                     interrupt_info = idle_interrupt_receiver.recv().await.unwrap_or_default();
                     info!(ctx, "smtp fake idle - interrupted")
                 }