fix lwt client interfaces

the desired interface is to use Lwt.Fail for errors hence the
option is extraneous
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)
mechanical changes to the tests to use new lwt interface
updates to a couple more tests to use the fixed lwt interface

in a separate commit because these also change the style to make
the tests more readable
Merge pull request #92 from rgrinberg/fix_lwt_interface

Fix lwt interface