MM-45993: Return errors during sending websocket messages (#20760)

During attaching an object to a websocket message, we would
marshal it to json and attach the string output. But if the
marshalling failed, we would just log a warning and move on.

This would add an empty string to the message. But the client
assumes that the object is correctly attached and would
fail silently if it cannot find it.

So we become more strict and return the error so that
it reaches the caller.

https://mattermost.atlassian.net/browse/MM-45993

```release-note
NONE
```

app/preference.go

@@ -9,7 +9,6 @@ import (
 	"net/http"
 
 	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 )
 
 func (a *App) GetPreferencesForUser(userID string) (model.Preferences, *model.AppError) {
@@ -69,7 +68,7 @@ func (a *App) UpdatePreferences(userID string, preferences model.Preferences) *m
 	message = model.NewWebSocketEvent(model.WebsocketEventPreferencesChanged, "", "", userID, nil)
 	prefsJSON, jsonErr := json.Marshal(preferences)
 	if jsonErr != nil {
-		mlog.Warn("Failed to encode to JSON", mlog.Err(jsonErr))
+		return model.NewAppError("UpdatePreferences", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(jsonErr)
 	}
 	message.Add("preferences", string(prefsJSON))
 	a.Publish(message)
@@ -103,7 +102,7 @@ func (a *App) DeletePreferences(userID string, preferences model.Preferences) *m
 	message = model.NewWebSocketEvent(model.WebsocketEventPreferencesDeleted, "", "", userID, nil)
 	prefsJSON, jsonErr := json.Marshal(preferences)
 	if jsonErr != nil {
-		mlog.Warn("Failed to encode to JSON", mlog.Err(jsonErr))
+		return model.NewAppError("DeletePreferences", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(jsonErr)
 	}
 	message.Add("preferences", string(prefsJSON))
 	a.Publish(message)