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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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
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• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
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• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
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• In-code documentation and comments are up-to-date.
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unittests/test_scanner.py

@@ -37,7 +37,8 @@ import yaml
 from caoscrawler.crawl import Crawler
 from caoscrawler.debug_tree import DebugTree
 from caoscrawler.scanner import (create_converter_registry, load_definition,
-                                 scan_directory, scan_structure_elements)
+                                 scan_directory, scan_structure_elements,
+                                 _load_definition_from_yaml_dict)
 from caoscrawler.structure_elements import (DictElement, DictListElement,
                                             DictTextElement, File)
 from pytest import raises
@@ -316,3 +317,22 @@ def test_record_parents():
             assert rec.parents[0].name == 'Stuff'  # default parent stays if no parent is given on
             # lower levels
             assert len(rec.parents) == 1
+
+
+def test_error_messages():
+    data = {
+        'Experiments': {}
+    }
+
+    broken_yaml = """
+EmptyConverter:
+    """
+    broken_definition = _load_definition_from_yaml_dict(
+        [yaml.load(broken_yaml, Loader=yaml.SafeLoader)])
+
+    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.
+