Keep the download queue moving after a failed download (#491)

When a download fails, manage_error_signal removes it from the queue but
doesn't send Go, so the next queued download never starts on its own.
Completed and Cancel both send Go; do the same here.

desktop/src-tauri/download_manager/src/download_manager_builder.rs

@@ -357,6 +357,7 @@ impl DownloadManagerBuilder {
         }
         self.push_ui_queue_update();
         self.set_status(DownloadManagerStatus::Error);
+        send!(self.sender, DownloadManagerSignal::Go);
     }
     async fn manage_cancel_signal(&mut self, meta: &DownloadableMetadata) {
         // If the current download is the one we're tryna cancel