workflows: js fixes and interface improvements

• Fixes JS inclusion issues on workflow admin pages as well as updating the blueprint with most recent API/model changes.

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

• Adds a new function to BibWorkflowObject that tries to return a well formatted representation of the data.

• Improves the formatting sequence when displaying data from workflow objects in Holding Pen to not cause exceptions when data is non-XML. Data is now shown as a pretty string when not formattable by formatter or legacy.bibfield.