NoComparisonDataError added

xsa/errors.py

@@ -38,6 +38,10 @@ class NoRestApiError(Error):
     """Should be raised when no REST-API-Interface was defined in the json-file of a root-type"""
     pass
 
+class NoComparisonDataError(Error):
+    """Should be raised when no comparision was defined in the json-file of a root-type"""
+    pass
+
 class CorruptedQueryError(Error):
     """Should be raised when the query-definition can not be loaded from file."""
     pass