DMN16 — Returning null is not enough for reporting/handling errors

Abusing null as an error indicator is not enough for reporting/handling errors.
In particular, it does not solve the need for users to:

• know exactly what the root cause of an error was
• control the reaction to errors

The specification already mentions error reporting but is neither a strict requirement nor consequently used:

When a built-in function encounters input that is outside its defined domain, the function SHOULD report or log diagnostic information if appropriate and SHALL return null.

Add strict mode

Improve de specification of error handling by adding two modes lenient and strict, with the lenient mode, the current one, the default one.

The idea is to configure the execution engine with the error handling mode when the models are executed.