Allow defining custom messages/pages for 404 Hosts

[pktiuk]

Is your feature request related to a problem? Please describe.

I would like to define some additional info in HTMLs returned by 404 hosts like (this website does not exist anymore because...)

Describe the solution you'd like

Adding simple form in 4040 host config

Describe alternatives you've considered

Changing HTML of default site, but it won't be sufficient

Additional context

It may be linked with #26