If a Definition contains multiple variable elements that all share the same delimiter,
escape character, etc, then those properties can be defined at the global level and
referenced in all of the element definitions rather than defining the same delimiter
character separately.
Provides an MDL specific Exception that can be used when there are errors
loading/serializing an MDL file, or when other errors occur that are specific
to MDL.
test get/add/remove elementDefinition.
Element definitions can be defined globally so they can be used by
multiple message or element definitions, or locally.