tracker/udp: Panic on read errors if ConnClient not closed

There's nothing exposed to handle this error currently, and if it occurs, it's better not to fail in silence. I'm currently debugging a situation where ConnClients stop working, this would be a condition we'd want to know about.

tracker/udp/conn-client.go

@@ -35,6 +35,9 @@ func (cc *ConnClient) reader() {
 			// TODO: Do bad things to the dispatcher, and incoming calls to the client if we have a
 			// read error.
 			cc.readErr = err
+			if !cc.closed {
+				panic(err)
+			}
 			break
 		}
 		err = cc.d.Dispatch(b[:n], addr)