### 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
this(message, -1);
        }
        public long timeout() {
            return timeout;
        }
        @Override
        public boolean isRawIndexingData() {
            return isRawIndexingData;
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            super.writeTo(out);
            out.writeString(message);
            out.writeLong(timeout);
        }
    }
    static class StringMessageResponse extends TransportResponse {
        private final String message;
        StringMessageResponse(String message) {
            this.message = message;
        }
        StringMessageResponse(StreamInput in) throws IOException {
            this.message = in.readString();
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            out.writeString(message);
        }
    }
    public static class Version0Request extends TransportRequest {