rss mention fix attempt; SITE_NAME is now required

app/lib/exceptions.py

@@ -0,0 +1,9 @@
+class MissingConfigurationException(Exception):
+    def __init__(self, configuration_field: str):
+        message = f"Missing configuration field '{configuration_field}'"
+        super().__init__(message)
+
+
+class SiteNameMissingException(MissingConfigurationException):
+    def __init__(self):
+        super().__init__('SITE_NAME')