Major Description
Layers need access to DFDL properties at layer compilation time, and for some (like delimiters) at runtime.
Layers need to be able to create Schema Definition Errors at compile time and at runtime, and need to be able to signal a variety of general Processing Errors.
(Currently, throwing LayerNotEnoughDataException is the only way to cause a processiing error. Other throws are all fatal.)
Layer length kind 'explicit' should be deprecated. It is not needed since one can create an element of specified length (lengthKind 'explicit', 'prefixed', 'pattern' or 'endOfParent' when parent is of specified length) surrounding a layer.
See also DAFFODIL-2843 and DAFFODIL-2844 which are also layer enhancements.
(Probably all should be fixed together along with firming up a SAPI Layer API)