Print stack trace on SIGUSR1
To assist with debugging issues that don't print an error. See https://stackoverflow.com/questions/132058/showing-the-stack-trace-from-a-running-python-application/133384#133384
To assist with debugging issues that don't print an error. See https://stackoverflow.com/questions/132058/showing-the-stack-trace-from-a-running-python-application/133384#133384