Update index.rst

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.
Merge pull request #9752 from pwaller/patch-1

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