Before this commit, escape sequences above std::char::MAX ('\u{10ffff}')
would overflow and cause a panic. This commit replaces an `unwrap` with
`ok_or` and a clearer error message. This message will likely change
in the future in order to better conform to the `dart-sass` implementation
which currently also fails to cleanly handle this overflow.
See https://github.com/kaj/rsass/pull/73
