streamline error messages

[emiltin]

The way errors are handled could be streamlined. The current spec has several different ways of reporting errors NotAck, age=undefined, etc. and it's not always clear when to use what.

[otterdahl]

I've opened PR #115 that can serve as a starting point

[emiltin]

thanks. step 1 can be more explanation and clarify about the current spec. step 2 could be a redesign of how we handle errors in core v4