Create task to reprocess failed documents

Apart from reprocessing the last failed document from the admin console, we should be able to do it from the console as a task. This can be defined as:

```
rake production app:reprocess_failed
```

Possible options could include (exclusively):
1. Process all failed docs when no arguments passed.
2. Document id argument
3. Organisation id argument
4. Account id argument
5. Last failed flag
