What does it mean to say that "no experiment is a failure"?

It is another way of saying, you can learn from you mistakes. If a experiment fails you can learn why and how it failed thus meaning you can prove something wrong or realize what mistake you made and learn not to do it again.

Hope this helps! :)

It is a way of saying that even if you mess up, or make a mistake, you can still learn from your failures to ensure that the same thing doesn't go wrong again. Basically, it's another way of saying that you learn from your mistakes.