Fail-fast behavior for topics with no available schemas

The latest refactorings by Chris seem to have removed the fail-fast behavior in KafkaAvroMessageDecoder which enabled the decoder to fail preemptively if it was being constructed for a topic that has no available schema in the registry.

Perhaps this is not the right place to put such fail-fast logic, in which case it should probably be handled somewhere else, because it seems to me like this is useful to have...

(Also added the target dirs in .gitignore)