Eio_unix.ErrSourceWrap Unix errors as Eio.Io ones.
v e fn arg returns an Eio.Io error corresponding to a Unix.Unix_error (e, fn, arg) exception.
Example:
try
Unix.rmdir "/tmp/foo"
with Unix.Unix_error (e, fn, arg) ->
raise (Eio_unix.Err.v e fn arg)