PIM-1775 Aggregate invalid data inside a downloadable file during product import

```
Bug fix: no
Feature addition: yes
Backwards compatibility break: yes
Unit test passes: yes
Behat scenarios passes: yes
Checkstyle issues: no
ChangeLog updated: no
Documentation PR: needed
Fixes the following jira:
 - PIM-1775
```
![capture du 2013-12-27 11 22 47](https://f.cloud.github.com/assets/668604/1814102/e12f4cc8-6ee0-11e3-90ac-72a36e19e5dd.png)