lwt_wrap_exn should take a `type t` argument
lwt_wrap_exn should take a `type t` argument

Before this patch we only passed in the filename, for logging
purposes. We want to check alignment invariants, so we will
need the full `t`.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/mirage-block-unix,samoht/mirage-block-unix,mirage/mirage-block-unix