workflows: fixes for admin interface

* Fixes JS inclusion issues on workflow admin pages as well
  as updating the blueprint with most recent API/model changes.
workflows: more exception improvements

* Adds another custom exception into the new 'errors' module
  (InvenioBibWorkflowWorkerUnavailable -> WorkflowWorkerError).

* Makes some cosmetic code-readabilty changes.
Merge pull request #129 from jalavik/workflows-update

workflows: fixes for admin interface