ImportExport: Easier debugging

Made the import more robust if import file links to a SKU which is skipped for some reasons -> leads to a "NULL" link causing fatal errors.

Steps to reproduce:

* Import file with SKU "1" in first line, but some errors (i.e. missing description)
* In second line SKU "2", valid product data, _links_upsell_sku contains the SKU "1"