public class AccessionNumberException
Exception thrown when an error occurs because an AccessionNumber was
invalid or used incorrectly.
Accession numbers are invalid under the following conditions:
1) INSERT mode, and the provided accesion # doesn't contain "sep"
2) UPDATE mode, and no accession # provided
3) UPDATE mode, and accession # doesn't exist in xml_documents table
4) DELETE mode, and no accession # provided
5) DELETE mode, and accession # doesn't exist in xml_documents table