### 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`)
server/src/main/java/org/elasticsearch/cluster/coordination/CoordinationDiagnosticsService.java
private final TimeValue nodeHasMasterLookupTimeframe;
    /**
     * If the master transitions from a non-null master to a null master at least this many times it starts impacting the health status.
     */
    private final int unacceptableNullTransitions;
    /**
     * If the master transitions from one non-null master to a different non-null master at least this many times it starts impacting the
     * health status.
     */
    private final int unacceptableIdentityChanges;
    /*
     * This is a Map of tasks that are periodically reaching out to other master eligible nodes to get their ClusterFormationStates for
     * diagnosis. The key is the DisoveryNode for the master eligible node being polled, and the value is a Cancellable.
     * The field is accessed (reads/writes) from multiple threads, but the reference itself is only ever changed on the cluster change
     * event thread.
     */
    
    volatile Map<DiscoveryNode, Scheduler.Cancellable> clusterFormationInfoTasks = null;
    /*
     * This field holds the results of the tasks in the clusterFormationInfoTasks field above. The field is accessed (reads/writes) from
     * multiple threads, but the reference itself is only ever changed on the cluster change event thread.
     */
    
    volatile ConcurrentMap<DiscoveryNode, ClusterFormationStateOrException> clusterFormationResponses = null;
    /*
     * This is a reference to the task that is periodically reaching out to a master eligible node to get its CoordinationDiagnosticsResult
     * for diagnosis. It is null when no polling is occurring.
     * The field is accessed (reads/writes) from multiple threads, and is also reassigned on multiple threads.
     */
    volatile AtomicReference<Scheduler.Cancellable> remoteCoordinationDiagnosisTask = null;