UrlGenerationError are not catched as 404 anymore

Fixes: https://github.com/rails/rails/issues/14185

Since `UrlGenerationError` inherits from `RoutingError`, it is handled as a 404 by default.

In my opinion it's not a good idea. Failing to generate an URL should be a 500 not a 404, and it should not be silently swallowed but reported to whatever error handler is present.
This should certainly not be updated as this is under the upgrading from 3.2 to 4.0 section ; the error was still `ActionController::Routing`.

However, this could be updated [here](https://github.com/rails/rails/blob/89397d09ebb7ca4089f17820d05d5eb223913652/guides/source/configuring.md#configuring-action-dispatch) in the `rescue_responses` code snippet. Thanks for your patch so far ! :+1: