various: resolve linter errors in a number of pkgs (#434)

- webui: resolve linter errors
- msghub: resolve linter errors
- policy: resolve linter errors
- extension: resolve linter errors

pkg/msghub/hub.go

@@ -120,7 +120,7 @@ func (hub *Hub) AddListener(l Listener) {
 		// Playback log
 		h.history.Do(func(v interface{}) {
 			if v != nil {
-				l.Receive(v.(event.MessageMetadata))
+				_ = l.Receive(v.(event.MessageMetadata))
 			}
 		})