fix: add error cause to connectivity view for IMAP errors

For SMTP errors we already format `last_send_error` with {:#},
but for IMAP errors we have formatted the errors with .to_string().
This resulted in errors such as
"Error: IMAP failed to connect to example.org:443:tls"
instead of
"Error: IMAP failed to connect to example.org:443:tls: Connection failure: Network is unreachable (os error 101)."
in the connectivity view HTML.

src/scheduler.rs

@@ -631,7 +631,7 @@ async fn smtp_loop(
             info!(ctx, "SMTP fake idle started.");
             match &connection.last_send_error {
                 None => connection.connectivity.set_idle(&ctx),
-                Some(err) => connection.connectivity.set_err(&ctx, err),
+                Some(err) => connection.connectivity.set_err(&ctx, err.clone()),
             }
 
             // If send_smtp_messages() failed, we set a timeout for the fake-idle so that