How to see the real error coming from Web API

link -> Custom errors and error detail policy in ASP.NET Web API | Jimmy Bogard's Blog:

Instead of relying on a lot of XML configuration, Web API uses a lot of programmatic configuration. This helps self hosting, but for changing policies like error detail, we have to change the code, re-compile and re-deploy. To set the error policy in our application, we need to modify our global Web API configuration

I was wondering the other day how to get the real error. I like the example he shows on how to read web.config and make it work like ASP.NET