use Lwt.Fail instead of None in client calls
use Lwt.Fail instead of None in client calls

- Implements the interface in the previous commit
- Currently the error returned are less than stellar (like before)

Drup/ocaml-cohttp,marklrh/ocaml-cohttp,inhabitedtype/ocaml-cohttp,edwintorok/ocaml-cohttp,mor1/ocaml-cohttp,dsheets/ocaml-cohttp,marklrh/ocaml-cohttp,fdopen/ocaml-cohttp,inhabitedtype/ocaml-cohttp,edwintorok/ocaml-cohttp,inhabitedtype/ocaml-cohttp,fdopen/ocaml-cohttp,dsheets/ocaml-cohttp,marklrh/ocaml-cohttp,Drup/ocaml-cohttp,dsheets/ocaml-cohttp,edwintorok/ocaml-cohttp,mor1/ocaml-cohttp,Drup/ocaml-cohttp,fdopen/ocaml-cohttp,mor1/ocaml-cohttp