csv import, add an option for not importing anything in case of error
if there s a an error during a csv import, a part of the users will be imported, another part no. to retry an import , its then necessary to find at which line the import failled (not displayed in the error message)
then either find all the users which were imported in fd delete them in fd and retry delete them from csv and retry
(from redmine: issue id 4675, created on 2016-04-04, closed on 2016-04-06)