Fix learn-ocaml build -j 2 issue

[erikmd]

This PR is intended to Reproduce and Fix #414

[erikmd]

Hi all,

I've tried 2 different fixes in this PR (tweak Lwt_utils.mkdir_p (in my reverted commit) or just calling Lwt_utils.mkdir_p once more).

Both fix the issue mentioned in issue #414: Avoid Cannot process exercises: Unix.Unix_error(Unix.EEXIST, "mkdir", "./www/static")

However, the issue is more pervasive than what I thought at first sight: despite this potential fix,
the parallel processing is still broken… because we get randomly, with the 2-exercises demo repositiory, one of these outcomes:

1. OK

Learnocaml v.0.12 running.
Updating app at ./www
demo2                        [OK]
demo                         [OK]

2. Partial build and freeze (the build process hangs):

Updating app at ./www
demo2                        [OK]

3. something yet more strange:

Learnocaml v.0.12 running.
Updating app at ./www
Grader error: Lwt.Resolution_loop.Canceled
demo                       [FAILED]
demo2                        [OK]
demo                         [OK]

So, I'm going to postpone this for the time being − fixing this parallel build issue would be nice, but will require more investigation in src/repo/learnocaml_process_exercise_repository.ml

(FTR, the outputs above mention "0.12" but the tested version was obviously the commits from this PR;
anyway, I think "the ambient version number" should be bumped in master anytime soon, and after each release)