Packages, error message when "isCustom" not provided

Purpose/Overview:
Endpoint PUT eholding/packages/{id}
If "isCustom" field is missing, server responds with error message like
"Attribute PackageName is missing."

The error message is not valid and entity doesn't even have an attribute "packageName".
So we need to improve error message.

Technical approach:

• Look why isCustom effected on name 
• Process situation when isCustom not provided
• Add field isCustom in documentation examples