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.
I fixed that and rebased, thanks!