Fix empty string on app error (#15166)

app/user.go

@@ -2172,7 +2172,7 @@ func (a *App) ConvertBotToUser(bot *model.Bot, userPatch *model.UserPatch, sysad
 
 	appErr := a.Srv().Store.Bot().PermanentDelete(bot.UserId)
 	if appErr != nil {
-		return nil, model.NewAppError("ConvertBotToUser", "", nil, err.Error(), http.StatusInternalServerError)
+		return nil, model.NewAppError("ConvertBotToUser", "app.user.convert_bot_to_user.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}
 
 	return user, nil