Hitting KeyboardInterrupt produces too many error logs

[ultmaster]

When hitting keyboard interrupt (via Ctrl+C for example), there is a lot of tracebacks on the console (particularly when there are a lot of runners). Something like asyncio.CancelledError...

To reproduce, run this file: https://github.com/microsoft/agent-lightning/blob/main/tests/benchmark/benchmark_store.py

I think this part can be greatly simplified and only show the important messages so that the real errors won't be flushed by the tons of logs.