Provide a way to override "prepareError()" to prepare meaningful errors in one place and handle 'jvm fatal' exceptions

[lobanovdmitry]

Default static implementation of "prepareErrors" in

• com.salesforce.reactorgrpc.stub.ServerCalls
• com.salesforce.rx3grpc.stub.ServerCalls
• com.salesforce.rxgrpc.stub.ServerCalls
  can't be customized. But sometimes it is very helpful:

1. You can define generic exception mapping on a service level in one place instead of setting it in every method.
2. Handle "JVM fatal" exceptions that are not propagated to onError reactive operator.

By overriding "prepareError" method, users can put some custom logic of handling in a service. For example:

new ReactorGreeterGrpc.GreeterImplBase() {
    @Override
    public Mono<HelloResponse> sayHello(Mono<HelloRequest> reactorRequest) {
        return reactorRequest.map(this::map);
    }

    private HelloResponse map(HelloRequest request) {
        // some critical exception can happen anywhere
        throw new NoSuchMethodError("Fatal!");
    }

    @Override
    protected Throwable onErrorMap(Throwable throwable) {
        // Override can contain any logic that will be executed in case of error
        if (throwable instanceof NoSuchMethodError) {
            return Status.INTERNAL.withDescription("NoSuchMethod:" + throwable.getMessage()).asRuntimeException();
        }
        return super.onErrorMap(throwable);
    }
};

[rmichela]

Thanks! This looks great. Let me get one more set of eyes. @nikolay-pshenichny Can you please take a look?