BibTasklet: Arxiv-DOI-update silent errors

* Schedule with BibTasklet, pass argument with -a
  skip_result_types=missing to ignore missing DOI errors.

* Possible DOI Errors: missing, ambigous, incorrect.

* By default, missing DOI messages are quietly ignored.

Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>