onError not called for POST requests

[kito99]

As things currently stand, I don't see how the onError() callback would ever be called for post requests, since handler defined in AbstractGraphQLHttpServlet always catches the exception and sets the response code to 500:

        this.postHandler = (request, response) -> {
            GraphQLInvocationInputFactory invocationInputFactory = configuration.getInvocationInputFactory();
            GraphQLObjectMapper graphQLObjectMapper = configuration.getObjectMapper();
            GraphQLQueryInvoker queryInvoker = configuration.getQueryInvoker();

            try {
...
            } catch (Exception e) {
                log.info("Bad POST request: parsing failed", e);
                response.setStatus(STATUS_BAD_REQUEST);
            }
        };

When the handler is called, it'll never reach onError(); it actually calls onSuccess() instead:

    private void doRequest(HttpServletRequest request, HttpServletResponse response, HttpRequestHandler handler, AsyncContext asyncContext) {

        List<GraphQLServletListener.RequestCallback> requestCallbacks = runListeners(l -> l.onRequest(request, response));

        try {
            handler.handle(request, response);
            runCallbacks(requestCallbacks, c -> c.onSuccess(request, response));
        } catch (Throwable t) {
            response.setStatus(500);
            log.error("Error executing GraphQL request!", t);
            runCallbacks(requestCallbacks, c -> c.onError(request, response, t));
        } finally {
            runCallbacks(requestCallbacks, c -> c.onFinally(request, response));
            if (asyncContext != null) {
                asyncContext.complete();
            }
        }
    }

Having this fixed would help with #129.