Add bind_error to Lwt_result

[bn-d]

There is map vs map_error and there is bind_lwt vs bind_lwt_error. I think having a bind_error would really make sense. This is very usefully when your processes have retry, backup/alternative loading method etc.

Thanks

[raphael-proust]

The changeset looks good to me. The history can use some squashing. Do you want to do it? Otherwise I can squash and push on your branch. Let me know.

[bn-d]

Let's just squash and push through GitHub. Thanks