do not expect on unbounded send (#2530)

polkadot/node/metered-channel/src/unbounded.rs

@@ -140,7 +140,7 @@ impl<T> UnboundedMeteredSender<T> {
 
     /// Attempt to send message or fail immediately.
     pub fn unbounded_send(&mut self, msg: T) -> result::Result<(), mpsc::TrySendError<T>> {
-        self.inner.unbounded_send(msg).expect("Unbounded send never fails. qed");
+        self.inner.unbounded_send(msg)?;
         self.meter.fill.fetch_add(1, Ordering::SeqCst);
         Ok(())
     }