That's not a problem at all, I already use prompts that allow the LLM to say they don't know an answer, and it does take that option when it's unable to find a correct answer. For instance I often phrase questions like this "Is it known whether or not red is a color in the rainbow?" And for questions where it doesn't know the answer it now will tell you it doesn't know.
And to your other point, the systems may not be capable of discerning their own hallucinations, but a totally separate LLM will be able to do so pretty easily.