[Feature request] Throw error when callback doesn't return a promise

[jvdlaar]

    $subscription = $connection->connectToPersistentSubscriptionAsync(
        'foo-bar',
        'test-persistent-subscription',
        function (EventStorePersistentSubscription $subscription, ResolvedEvent $event, int $retry) use ($stopWatch, &$i)/*: Promise*/ {
            echo 'incoming event: ' . $event->originalEventNumber() . '@' . $event->originalStreamName() . PHP_EOL;
            echo 'data: ' . ($event->originalEvent() ? $event->originalEvent()->data() : '<null>') . PHP_EOL;
            echo 'retry: ' . $retry . PHP_EOL;
            echo 'no: ' . ++$i . ', elapsed: ' . $stopWatch->elapsed() . PHP_EOL;

            //return new Success();
        },
        function () {
            echo 'dropped' . PHP_EOL;
        },
        10,
        true,
        new UserCredentials('admin', 'changeit')
    );

Above example doesn't return Success. An error should be thrown in such situation because the callback doesn't return a promise.

What happens now is:

Amp\ByteStream\StreamException: The stream was closed by the peer

/app/vendor/amphp/byte-stream/lib/ResourceOutputStream.php:178
/app/vendor/amphp/byte-stream/lib/ResourceOutputStream.php:141
/app/vendor/amphp/socket/src/Socket.php:69
/app/vendor/prooph/event-store-client/src/Transport/Tcp/TcpPackageConnection.php:171
/app/vendor/amphp/amp/lib/Coroutine.php:39
/app/vendor/amphp/amp/lib/Loop/Driver.php:119
/app/vendor/amphp/amp/lib/Loop/Driver.php:70
/app/vendor/amphp/amp/lib/Loop.php:76
/app/src/Subscription/TcpCompetingConsumer.php:121
/app/tests/Subscription/TcpCompetingConsumerTest.php:224

Because the server will not send the heartbeat response.