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

@@ -335,3 +335,17 @@ EmptyConverter:
     with pytest.raises(RuntimeError, match="Definition of converter \"EmptyConverter\" is empty"):
         scan_structure_elements(DictElement(name="", value=data), broken_definition, converter_registry)
 
+    broken_yaml = """
+Converter:
+  type: DictElement
+  records:
+    TestRecord: "42"
+    """
+
+    broken_definition = _load_definition_from_yaml_dict(
+        [yaml.load(broken_yaml, Loader=yaml.SafeLoader)])
+
+    converter_registry = create_converter_registry(broken_definition)
+
+    with pytest.raises(RuntimeError, match="dict expected, but found str: 42"):
+      scan_structure_elements(DictElement(name="", value=data), broken_definition, converter_registry)