log response body in case of handler failure

In ran into an ocaml-github bug and not seeing the returned body made
the debugging more difficult -- well, I implemented this. You may want
to tweak this patch:

- it is not clear that the right thing to log is the body in full;
  in my case (failure of a Json parse because of a
  not-optional-by-mistake field), catching the error at the level of
  the Json parsing, re-running the parser on the pretty-printed body,
  and giving me the pretty-printed body would have make my life easier
  (I did just that by hand to get a sensible line number in the error
  message)
- implementation-wise I used two nested try_lwt as I defensively
  assumed that CLB.string_of_body may fail as well (ah, let-try, how
  we miss thee...)