workflows: holdingpen details page improvements

* Adds 2 new stages for workflow objects error and waiting.

* Better display of workflow definition.

* Display of completed tasks.
This line will cause `KeyError: '_task_history' ` for instances with existing records in Holding Pen. Can you look into it, perhaps by adding a new function to `BibWorkflowObject` alongside `update_task_history(self, ..)` to `get_task_history(self)` that will use the `dict.get` function.