Add blocking mode that displays an errorpage

Sorry if duplicate - I couldnt find a feature request for this.

Not many people understand when their browser yells NXDOMAIN at them, so they raise questions.

- Present user a Blockingpage pulled from the webserver that adguard runs anyway.

- Show the user what filterled to his/her block and maybe give ability to (request a) whitelist the domain.

- Maybe even more aditional statistics about the client.

- Evaluate performance impact/overhead of serving those pages to X amount of clients.