Closes #1122 (closed). Uploads will get stuck in status PENDING if adding the celery task to the queue fails (for example if rabbitmq is down). This MR adds proper error handling (setting the Proc to status FAILED if the @process-task could not be added to the queue), and also updates the celery version to get more consistent behavior of the celery framework (the currently used version did not always raise an exception as expected).