PIM-3446: limit the warning display to 100 in import/export

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3446
| DB schema updated?   | N
| Migration script?    | N
| Doc PR               | N

The limit displayed in the results
http://dl.dropbox.com/u/41163683/S%C3%A9lection_005.png
wow, I really don't like the fact that you reuse $warning in this foreach !