Added customization_help() to Source, as calibre-customize was failing on builtin Metadata source plugins.

src/calibre/ebooks/metadata/sources/base.py

@@ -212,6 +212,9 @@ def is_configured(self):
     def is_customizable(self):
         return True
 
+    def customization_help(self):
+        return 'This plugin can only be customized using the GUI'
+
     def config_widget(self):
         from calibre.gui2.metadata.config import ConfigWidget
         return ConfigWidget(self)