[WIP] Implement @error function

In reference to https://github.com/sass/sass-spec/issues/136 and https://github.com/sass/libsass/issues/704

I first tried to duplicate all `Warning` related code, but ran into the issue that `Error` and `SASS_WARNING` are already taken by other parts. Therefore I decided to "hack" `Warning` to add a `is_fatal` flag to indicate these kind of Errors internally. I guess this does make sense, although the namings are already a bit confusing and this PR does not help to reduce that. This is not complete as this should also be added to the C Interface (related to #711).


This is how ruby sass prefixes them! ¯\_(ツ)_/¯