zulip: Treat unknown errors with a 10-second backoff.

An unknown error (including an unauthorized error) would fall through
with no calls to time.Sleep, resulting in hammering the server as
quickly as possible.

Add a 10-second sleep in the default error case.  The heartbeat is
left with no explicit sleep, but all other codepaths now contain one.

bridge/zulip/zulip.go

@@ -125,6 +125,7 @@ func (b *Bzulip) handleQueue() error {
 			b.Log.Debug("heartbeat received.")
 		default:
 			b.Log.Debugf("receiving error: %#v", err)
+			time.Sleep(time.Second * 10)
 		}
 		if err != nil {
 			continue