Improve error messages and error handling on DMN compilation and execution

This is a placeholder ticket for several small improvements on the error messages and error handling.

1. When a decision or a BKM has no expression, it raises the message “No expression defined for node ‘_guid’”. Would it be possible to put the node name when one is defined instead?

2. Add the DMN model object reference to the DMNMessage class.

3. Avoid NPE and NoSuchElementException when a function or decision table returns null as result.