### 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
serviceA.registerRequestHandler(
            "internal:localNode",
            ThreadPool.Names.GENERIC,
            StringMessageRequest::new,
            (request, channel, task) -> {
                try {
                    channel.sendResponse(new StringMessageResponse(request.message));
                } catch (IOException e) {
                    exception.set(e);
                }
            }
        );
        final AtomicReference<String> responseString = new AtomicReference<>();
        final CountDownLatch responseLatch = new CountDownLatch(1);
        serviceA.sendRequest(
            nodeA,
            "internal:localNode",
            new StringMessageRequest("test"),
            new TransportResponseHandler<StringMessageResponse>() {
                @Override
                public StringMessageResponse read(StreamInput in) throws IOException {
                    return new StringMessageResponse(in);
                }
                @Override
                public void handleResponse(StringMessageResponse response) {
                    responseString.set(response.message);
                    responseLatch.countDown();
                }
                @Override
                public void handleException(TransportException exp) {