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.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• 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)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.

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
@@ -318,6 +319,40 @@ def test_record_parents():
             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)
+
+    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)
+
+
 def test_units():
     """Test the correct setting of units."""
     crawler_definition = load_definition(UNITTESTDIR / "test_unit_cfood.yml")