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>
We could, but it would not bring much improvements as it anyways may be overwritten by the parameter. 

Something like?
```python

if oai_namespace:
     nsmap[None] = oai_namespace
else:
     nsmap = NSMAP   # where NSMAP = {None: "http://www.openarchives.org/OAI/2.0/"}
```