should fix #2320

Sources::Site#normalize_for_artist_finder! catches errors that occur
during the normalization process. However it only catches
StandardErrors, not Exceptions, so this makes Sources::Error a
StandardError.

app/logical/sources/error.rb

@@ -1,4 +1,4 @@
 module Sources
-  class Error < Exception
+  class Error < StandardError
   end
 end