The change in 1.4 to log unhandled messages in handle_info is, in my opinion, a good one, that will help many developers in uncovering some of the issues in their code bases and libraries they use.
That said, I'm not convinced that logging with error level is the right choice. It creates confusion in many places when things that are silent on 1.3 start suddenly "throwing errors" on 1.4. While I understand that those are not exceptions, from couple questions I've already seen here and there, it's not entirely obvious to many developers and they consider their apps "broken" because of that.
Maybe the log level should be revisited on this one? I think a warning instead of an error would be much more appropriate, at least initially - it could be switched to error in the next release, when people are more familiar with that issue and libraries are properly adjusted, and leaking messages are uncovered.