Is there a way to detect improperly formatted webvtt caption file?

Using the AVAssetResourceLoaderDelegate, we specify the URL to our webvtt caption file in order to get "baked in" captions that will display externally if the user switches to AirPlay. The problem I'm running into is - if we somehow have a .vtt captions file that is not formatted correctly no captions display on the screen and, from all the digging I've done, the AVPlayer or AVAssetResourceLoaderDelegate are not alerted to the fact that there is any error. Is there anywhere to be alerted if we have a bad captions file, so we know captions are not displaying?