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"
}
```