workflows: holdingpen details page improvements

* Adds 2 new stages for workflow objects error and waiting.

* Better display of workflow definition.

* Display of completed tasks.
Wouldn't be nicer to have somewhere a mapping defining message and `class` for each version state to use it also in https://github.com/inveniosoftware/invenio/pull/2331/files#diff-f8f318ffa0bca742f5220fa150ab04c8R33 ?

```
HOLDINGPEN_WORKFLOW_STATES = {
    versions.HALTED: {'message': _('Halted'), 'class': 'success'},
    ...
}
```