We now consider debugerrors harmful: we've run into cases where
debugerror.Wrap() hid valuable error information (err == io.EOF?).
I've removed them from the main code, but left them in some tests.
Go errors are lacking, but unfortunately, this isn't the solution.

It is possible that debugerros.New or debugerrors.Errorf should
remain still (i.e. only remove debugerrors.Wrap) but we don't use
these errors often enough to keep.