Raise ValidityException (und subclasses) on validity violations

[franzlst]

Instead or raising builtin Python exceptions (ValueError, etc), custom RAFCON ValidityException should be raised in their proper subclasses (e.g. DataPortTypeMismatchException).

This would e.g. allow to write more future-safe code then e.g. the following example from state_element.py: if "not have matching data types" in message:

Originally created by @franzlst ([email protected]) at 2019-07-15 13:16:42+00:00 (moved from RMC internal repository)