CompletionStage behaviour in case of WebApplicationException

Hello,

I'm faced with a case where I'm not sure what the behavior should be.
When throwing a WebApplicationException, jersey automatically translates the exception in its corresponding response.

While using CompletionStage, I would expect the behavior to be similar. In practice, it translates to an internal server error. The JAX-RS specification is not very clear on that particular point

Consider the following case :

```
@GET
@Path("failAsync/later")
public CompletionStage failAsyncLater() {
CompletableFuture fail = new CompletableFuture<>();
failLater(fail, new IllegalStateException("Uh-oh"));

// We re-throw a WebApplicationException .exceptionally.
// This has the side effect of wrapping the WebApplicationException inside a CompletionException
return fail.exceptionally(ex -> {
throw new WebApplicationException("OOPS", Response.Status.NOT_ACCEPTABLE.getStatusCode());
});
}

private void failLater(CompletableFuture cf, Throwable t) {
new Thread(() -> {
try {
Thread.sleep(500);
cf.completeExceptionally(t);
}
catch (InterruptedException ex) {
// nothing to do, let the thread die.
}
).start();
}
```

I would actually expect to get an HTTP 406, instead of a 500.

Can you clarify the current behavior and the way this kind of error is supposed to be handled ?

Thanks a lot!

Start with the failAsyncLater CompletionStage example and reproduce how the wrapped WebApplicationException is translated. Read Jersey's asynchronous exception-handling path and the relevant JAX-RS specification text; done means the current behavior and the expected handling of the HTTP 406 response are clearly established.

Written by the indexing model from the issue text.