Allow treating a missing highlight language as error

[SamWilsn]

This is a tiny non-breaking feature. I already did the work before seeing the note about your forum in the pull request template. I hope I'm not being too rude by skipping making a post there 😅

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

[Keats]

Thanks!