Attribute type creation error handling

[GillesVanc]

When (re)running an import script, it would be nice if the client can decide if an attribute already exists. Only when needed, I want to create new attribute types. Now when creating attributes I need to handle 400 errors when an attribute type already exists. It would be much nicer if I call an add_attribute_type function. This function checks if this attribute type exists (or has not the same type or description) and only if needed he creates this attribute.