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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src/caoscrawler/converters/converters.py

@@ -375,6 +375,10 @@ class Converter(object, metaclass=ABCMeta):
         The `type` key in the `definition` defines the Converter class which is being used.
         """
 
+        if definition is None:
+            raise RuntimeError("Definition of converter \"{}\" is "
+                               "empty".format(name))
+
         if "type" not in definition:
             raise RuntimeError(
                 "Type is mandatory for converter entries in CFood definition.")