oaiharvester: authorlist extraction task

* Updates post-process task for authorlist extraction to
  properly fall back to the default XSLT stylesheet.

* Adds a check that the script file is found for the filtering
  script task.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
used in https://github.com/inveniosoftware/invenio/pull/2504/files#diff-f94d8d291a1bd6e2428c17989e81f2e2R55 shall we factor out to default constant?