uridownloader: Reset cancelled state after fetching an URI

Otherwise it will never ever be able to fetch anything after an
error until the user resets it manually.

gst-libs/gst/uridownloader/gsturidownloader.c

@@ -478,9 +478,12 @@ quit:
             "Failed to download '%s'", uri);
       } else {
         g_propagate_error (err, downloader->priv->err);
+        downloader->priv->err = NULL;
       }
     }
 
+    downloader->priv->cancelled = FALSE;
+
     g_mutex_unlock (&downloader->priv->download_lock);
     return download;
   }