Generic NPE error without details on product/variant import

**Describe the bug**
When we try to import a new product and one of the variants on the product has the same key the import fails but there is no detailed message as to why. The same is the case when we try to import a new product with an existing SLUG, there is always the same, generic error message. This was working before but seems like in some update of the library or on Commercetools side it just gives generic NPE.

This is an issue when we need to report to the calling system why did the import fail.

**To Reproduce**
Steps to reproduce the behavior:
1. Import a new product with the same Slug as an existing product
2. NPE happens without any details

**Expected behavior**
Error details

**Screenshots**