oaiharvest: reliability improvement and docs

* Prints not only error logs for every workflow, but also
  everything with a superior code error (40) like critical.

* Adds basic sphinx documentation syntaxes to various functions.

* Reactivates mails and tickets for harvesting process.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>