Hello,

after merging #215, I got some test failures of code that calls errors.Cause(err) where err is returned by some stdlib API: in my case err is returned by net.Conn.Write(), and errors.Cause(err) used to return a net.OpError instance, whereas it now returns a syscall.Errno instance.

I understand the reason for the change, but just wanting to highlight this is introduces backward-incompatible behavior, at least for some consumers.

YMMV re fixing this issue. Other people might find it useful to read, if the stumble upon the same regression.