Added the NewMasterVariantAdditionNotAllowed error code

A new error code, NewMasterVariantAdditionNotAllowed, is returned when an import operation fails because a newly imported Product Variant is set as the Master Variant (isMasterVariant=true).

Only an existing Product Variant can be set as the Master Variant. To resolve the error, you can either update the existing Master Variant or create a new Product Variant and then set it as the Master Variant.

Changes:

• [API] Added the NewMasterVariantAdditionNotAllowed error code.