Improve error handling on misconfigured connectors

Intro

The log simply states that the polling thread has died. It should mention that there is probably an earlier entry in the log explaining what caused the shutdown.

Reported issue

When the s3 source connector fails on startup (due to a wrong config?), the exception thrown is very generic. The output looks as shown below. It would improve usability to make the exceptions thrown more explicit.

{
  "name": "s3-source-connector",
  "connector": {
    "state": "RUNNING",
    "worker_id": "kafka-connect:8083"
  },
  "tasks": [
    {
      "id": 0,
      "state": "FAILED",
      "worker_id": "kafka-connect:8083",
      "trace": "org.apache.kafka.connect.errors.ConnectException: io.aiven.kafka.connect.s3.source.S3SourceTask polling thread has died\n\tat io.aiven.kafka.connect.common.source.AbstractSourceTask.poll(AbstractSourceTask.java:224)\n\tat org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.poll(AbstractWorkerSourceTask.java:462)\n\tat org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.execute(AbstractWorkerSourceTask.java:351)\n\tat org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:202)\n\tat org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:257)\n\tat org.apache.kafka.connect.runtime.AbstractWorkerSourceTask.run(AbstractWorkerSourceTask.java:75)\n\tat org.apache.kafka.connect.runtime.isolation.Plugins.lambda$withClassLoader$1(Plugins.java:181)\n\tat java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)\n\tat java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)\n\tat java.base/java.lang.Thread.run(Thread.java:829)\n"
    }
  ],
  "type": "source"
}