If projet import is too long, there's no message except "Les données n'ont pu être être chargées à cause d'un délai d'attente" (in french). There should be a confirmation of the request of the processing instead of the error message, and a track in log.
We have added a lot more functionality for import/export manager in 16.0. Also, 16.1 is shipping in May/2017. Please upgrade to either 16.0 or 16.1. If you are still receiving an unexpected error, it’s best you open a support ticket with SupportLine so we can properly resolve this. Thank you.