Better error management of the session during the upload progress

[francbartoli]

It is claiming with an internal server error message:

Internal Server Error: /upload/progress
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, callback_args, *callback_kwargs)
  File "./geonode/upload/views.py", line 105, in data_upload_progress
    upload_session = req.session[upload_id]
  File "/usr/local/lib/python2.7/site-packages/django/contrib/sessions/backends/base.py", line 48, in _getitem_
    return self._session[key]
KeyError: '4'