Is this a new feature?
Is this new? I don't remember seeing anything like it before.
It's not a new feature, but it's not commonly used. Course creators can provide such specialized explanations for some common predictable mistakes. Both of the mistakes you made look like something you could expect a learner to do, and therefore the team added an explanation.
Here's an example someone found in the Esperanto course: http://i61.tinypic.com/awf0b4.png but such messages exist in some other courses as well.