Share: Improve handling of URLs that have unsupported chapter - variant combination

[anubh-v]

Currently, the playground will use the default variant if the the URL contains an unsupported chapter - variant combination.

Example: Chapter = 4, and variant = non-det

As mentioned in this comment, this may not be the best approach.

My view: For such URLs, we can use default variant but there should be a warning shown to the user.

[martin-henz]

I think this proposal is simple and clear. Let's go ahead with it.