Handling spring reactor exceptions in imperative spring application

Question

I'm using the webflux in an imperative spring boot application. In this app I need to make rest calls to various backends using webclient and wait on all the responses before proceeding to the next step.

ClassA

public ClassA
{
     public Mono<String> restCall1()
     {
          return webclient....exchange()...
                 .retryWhen(Retry.backoff(maxAttempts, Duration.ofSeconds(minBackOff))
                 .filter(this::isTransient)
                 .onRetryExhaustedThrow((retryBackoffSpec, retrySignal) -> {
                     return new MyCustomException();
        });
     }
}

ClassB

public ClassB
{
     public Mono<String> restCall2()
     {
          return webclient....exchange()...
                 .retryWhen(Retry.backoff(maxAttempts, Duration.ofSeconds(minBackOff))
                 .filter(this::isTransient)
                 .onRetryExhaustedThrow((retryBackoffSpec, retrySignal) -> {
                     return new MyCustomException();
        });
     }
}

Mono<String> a = classAObj.restCall1();
Mono<String> b = classBObj.restCall2();

ArrayList<Mono<String>> myMonos = new ArrayList<>;

myMonos.add(a);
myMonos.add(b);

try {
List<String> results = Flux.mergeSequential(myMonos).collectList().block();} 
catch(WebclientResponseException e) {
.... 
}

The above code is working as expected. The Webclient is configured to throw error on 5xx and 4xx which I'm able to catch using WebclientResponseException.

The problem is I'm unable to catch any exceptions from the react framework. For example my web clients are configured to retry with exponential backoff and throw exception on exhaustion and I have no way to catch it in my try catch block above. I explored the option to handle that exceptiom in the webclient stream using onErrorReturn but it does not propagate the error back to my subscriber.

I also cannot add the exception to the catch block as it's never being thrown by any part of the code.

Can anyone advice what is the best way to handle these type of error. scenarios. I'm new to webflux and reactive programming.