Upgrade slow send restart of get_updates requests to blueslip.error.

Now that this is no longer a known problem with our product, we want
to hear about it when it happens.

I worry a bit that a 2s fuse may be too aggressive for the case of
customers in Europe, but it might be OK.

(imported from commit d1bd6b85cd8dffab9c0d0fd410de5331736b00af)

static/js/compose.js

@@ -466,7 +466,7 @@ function send_message(request) {
         $("#sending-indicator").hide();
         setTimeout(function () {
             if (exports.send_times_data[message_id].received === undefined) {
-                blueslip.debug("Restarting get_updates");
+                blueslip.error("Restarting get_updates due to delayed receipt of sent message " + message_id);
                 restart_get_updates();
             }
         }, 2000);