### Description

The `sendRequest` method is asynchronous and notifies a 
`TransportResponseHandler` when it completes the excecution
(successfully or unsuccessfully). 

When opening the connection we only catch `NodeNotConnectedException`
( https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/transport/TransportService.java#L719 ) which is currently fine as no implementations
throw other type of exception however, it'd be best to catch `Exception` here
so the `handler` gets notified of potential exceptions (as opposed to the caller
having to `try/catch` the call to `sendRequest`)
test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java
}
        }
        serviceB.registerRequestHandler(
            "internal:action1",
            randomFrom(ThreadPool.Names.SAME, ThreadPool.Names.GENERIC),
            TestRequest::new,
            new TestRequestHandler(serviceB)
        );
        serviceC.registerRequestHandler(
            "internal:action1",
            randomFrom(ThreadPool.Names.SAME, ThreadPool.Names.GENERIC),
            TestRequest::new,
            new TestRequestHandler(serviceC)
        );
        serviceA.registerRequestHandler(
            "internal:action1",
            randomFrom(ThreadPool.Names.SAME, ThreadPool.Names.GENERIC),
            TestRequest::new,
            new TestRequestHandler(serviceA)
        );
        int iters = randomIntBetween(30, 60);
        CountDownLatch allRequestsDone = new CountDownLatch(iters);
        class TestResponseHandler implements TransportResponseHandler<TestResponse> {
            private final int id;
            private final String executor = randomBoolean() ? ThreadPool.Names.SAME : ThreadPool.Names.GENERIC;
            TestResponseHandler(int id) {
                this.id = id;
            }
            @Override
            public TestResponse read(StreamInput in) throws IOException {