core: better error on export of internal type

more descriptive error when trying to export OrType, NoneOr, ValueType
and DataTypeType

Change-Id: If13815e9d2b177042b24a1bb62b1ad1d1d88b502
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/32737
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

test/test_datatypes.py

@@ -41,7 +41,7 @@ def copytest(dt):
 
 def test_DataType():
     dt = DataType()
-    with pytest.raises(NotImplementedError):
+    with pytest.raises(ProgrammingError):
         dt.export_datatype()
     with pytest.raises(NotImplementedError):
         dt('')