Add Custom Block page

[FrancescoFdd5]

Prerequisites

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• [v ] I am running the latest version
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I would like to ask if it was possible that in the next versions the possibility of returning an error page in case of blocked url is added.
And to add the option to put a url as an error page

Like this one: https://user-images.githubusercontent.com/29142494/74101828-250ebd80-4b3e-11ea-97d0-be3a9cc6ec1b.png

[ameshkov]

Here's how it's recommended to do now:
#454 (comment)

For now, we don't plan to go further and implement our own TLS pixel.