Provide a way for video providers to validate input.

Many video providers interact with an external API to retrieve data about a video, like thumbnails. If there's a failure in obtaining that required information, like if the video doesn't exist or the HTTP call fails for some reason, there's nothing a video provider can do to properly communicate that issue. The only recourse is to throw a fatal error and log something.

Seems like there's a better way to handle errors in a graceful way. I think ideally each provider would have a "validate" method which would be used to validate that the input can be used. We already have the static isApplicable method, but I think this is just an appropriate place to do text string analysis of the input. The "validate" method can do HTTP calls to make sure proper responses are returned.

Then, a field constraint can be added to the VideoEmbedField field which runs this validation logic for the selected provider.

The main problem I see is that this would be an API break, because you'd need to add a method to the interface. You could a default implementation in the abstract class but I don't think you can assume that every contrib provider extends the abstract base implementation.