Gracefully handle errors during big CSV import

Steps to reproduce

1. Install nuxeo-csv addon
2. Go to a workspace in Nuxeo WebUI
3. Click on the + button then go to the CSV tab
4. Choose the attach CSV
5. Note that the import is in progress for the 11000+ first documents whereas the counter does not indicate how many total lines should be imported
6. When the first error occurs, an error is displayed in the popup

Expected behavior:

Nuxeo should gracefully handle the errors during import and display a report with the errors at the end.

Moreover it could be interesting to display the total number of lines during the import.