A beginner is going to have a bad time with any language. Sure, 'nyi is not very descriptive, but an error message is not the place to teach total beginners. Unless you put some effort into learning a language (or already know something similar), compiler errors will be difficult to figure out anyway.
The most blatant example is the infamous "discards qualifiers" error but, for example, in a programming language with nice and descriptive error messages like Go, you will have a hard time figuring out what a interface type error is about until you actually read the manual and learn what an interface is.
A good error message doesn't have to be descriptive, but if it isn't, it absolutely must be Googleable -- i.e. unique enough so that someone who doesn't understand what it means can paste it into a search engine and find out.
If I Google "'nyi", I get a bunch of results about the New York Islanders hockey team. Not helpful. If I try to be more specific and Google "'nyi k7", the closest thing to a helpful result I get is a link to this discussion on HN.
