This handler is used to give the mapped custom fields an opportunity to validate the custom field values
that we are going to ask them to map. The custom fields will be provided with the values context
NOTE: The mapper that is passed to this class MUST be a mapper that has either categorized EVERY custom field
referenced by the custom field values as mapped or ignored. The class will throw ParseException if it encounters
a custom field that is not mapped in one of these two states.
This is the main method to implement when using this ImportEntityHandler. This method will provide the
entity name and a complete map of attribute key/value pairs. This includes any nested element tags that
will have CDATA bodies.