Decorate GitRepository.is_whitelisted() with retry_on_errors

See:
- http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.0-merge-push/68/console
- http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-push/307/console
- http://ci.openmicroscopy.org/view/Docs/job/OMERO-5.1-merge-docs/144/console
- http://ci.openmicroscopy.org/view/Failing/job/BIOFORMATS-5.1-merge-openbytes-performance/166/console