Design, implement error handling stratgey when encrypt/decrypt throw exceptions

[chris-giblin]

As per discussion of encmod/src/main/java/io/strimzi/kafka/topicenc/EncryptionModule.java in PR #9 the current impl catches as logs exceptions occurring during encryption/decryption.

This placeholder impl should be changed to throw exceptions which ultimately will be caught in MessageHandler::handle(Buffer buffer) where a Kafka error response must be returned to the requesting Kafka client.