Better error message

Summary

This very small MR just adds two more verbose error messages for two mistakes that can appear during cfood development.

Please see the in-code comments for details.

Test Environment

There are unit tests.

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unittests/test_scanner.py

@@ -332,7 +332,6 @@ EmptyConverter:
 
     converter_registry = create_converter_registry(broken_definition)
 
-    scan_structure_elements(DictElement(name="", value=data), broken_definition, converter_registry)
-
-    # TODO: this already raises the correct error message. This needs to be checked.
+    with pytest.raises(RuntimeError, match="Definition of converter \"EmptyConverter\" is empty"):
+        scan_structure_elements(DictElement(name="", value=data), broken_definition, converter_registry)