Make the /harvest page more robust if source.status is not set

This prevents exceptions from appearing in the log from Jinja:
```
     [error] [client 1.2.3.4] Error - <class 'jinja2.exceptions.UndefinedError'>: 'dict object' has no attribute 'status'
```

The described exception leads to a 500 - Server Error page to appear, hence you are unable to see you harvest sources and can't modify them.