Add warning section for `rescue_from Exception` in Guides

Now that we have Custom error page in Guides, we should add this warning as well. I've seen `rescue_from Exception` or `rescue_from StandardError` a lot of times. In any case, it's for custom error pages.
As documented, it causes serious side-effects such as:

  * You won't be able to see exception details and tracebacks during development.
  * Rack middleware that expects exceptions to hit it(e.g. airbrake) will stop working.

I also think that we should at least output a warning when `rescue_from` is given `Exception` / `StandardError` or we shouldn't allow to do so as I've never seen any proper use case for it. What do you think?