When an error occurs during CSV import of attendees, report errors

**Describe the bug**
I created a CSV file with a list of attendees. I imported this file to AlfIO. The progress bar reported 299 imported on 300. Indeed, one entry

**To Reproduce**
Steps to reproduce the behavior:
1. Go to Actions > import existing attendess
2. Create a simple CSV file with 3 lines and re-use the very same "reference" value twice
3. Click on import, the preview list should display the 3 entries
4. Click on "Upload" and the progress bar should show "2/3" importedd

**Expected behavior**

I expect to see csv errors reported directly on the web page. Maybe just the java exception, but something that would notify the user, that the file is not valid.

Some errors are reported on the console log. This is how I found the error and fixed my CSV file. It would be useful to see those errors on the web page.