Small doc fix: don't teach people to silence ImportError

Fix a pet peeve of mine so that beginners don't repeat the mistake: silencing `ImportError` is evil since you don't know if it came because `dependency_that_sometimes_exists` is unavailable or the `dependency_that_sometimes_exists` tried to import something which was broken.

This can seriously hinder debugging and I think it's a bad thing to teach it to people through documentation.