Improve .../urlfilter/... error handling as well as our filter UI

Packages from the `github.com/AdguardTeam/urlfilter` module currently often swallow rule parsing errors, which is rather undesirable for us, as we want to at least print them to log. That module must be improved to allow us to handle rule parsing errors better.

Also, in our UI we really need some form of validation as well. For example, drawing the “squiggly lines” under the obviously incorrect rule lines.