camel-kafka: better error messages in the health check

The message from the root exception is currently not added to the Health Check, which results in errors like "KafkaConsumer is not ready" without any more additional information.

We could extract the message from the root cause, which includes details on why the KafkaConsumer is not ready, like an unreachable bootstrap url, or an invalid port number.

Also, in cases where pollExceptionStrategy was not able to be set, a NPE is thrown during the health check, which results in a bad message.