[FEATURE] Include the root exception cause within the ExecutionException thrown by the OpenSearchAsyncClient

Is your feature request related to a problem?

The OpenSearchAsyncClient is used to perform a GetIndicesSettingsRequest below to retrieve the indices settings for a particular index.

CompletableFuture<GetIndicesSettingsResponse> getIndicesSettingsResponse = sdkJavaAsyncClient
            ._transport()
            .performRequestAsync(getIndicesSettingsRequest, GetIndicesSettingsRequest._ENDPOINT, TransportOptions.builder().build());

In the case that the requested index is not found, the exception thrown is the java.util.concurrent.ExecutionException, which includes an inner exception cause of type org.opensearch.client.opensearch._types.OpenSearchException. The OpenSearchException does not include an inner cause, however it does provide the message : Request failed: [index_not_found_exception] no such index [.opendistro-anomaly-detector-jobs].

The full logs can be seen here :

00:28:12.909 [opensearch[ad-extension][generic][T#4]] INFO  org.opensearch.ad.indices.AnomalyDetectionIndices - Exception e : class java.util.concurrent.ExecutionException
00:28:12.909 [opensearch[ad-extension][generic][T#4]] INFO  org.opensearch.ad.indices.AnomalyDetectionIndices - Exception e.getCause() : class org.opensearch.client.opensearch._types.OpenSearchException
00:28:12.909 [opensearch[ad-extension][generic][T#4]] INFO  org.opensearch.ad.indices.AnomalyDetectionIndices - Exception e.getCause().getMessage() :Request failed: [index_not_found_exception] no such index [.opendistro-anomaly-detector-jobs]

What solution would you like?

The OpenSearchException.getCause() should return the actual exception thrown, not just the exception message. In this case, the expected exception should be IndexNotFoundException.