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>
True that it could be a config actually (the default namespace..) :+1: