Make sure that buffered cursors are committed in case of event processing failure

[mtesseract]

If the callback provided to subscriptionProcess fails with an exception, event processing stops.

In this case, all buffered cursors up to (excluding) the failing event batch should be committed right before subscriptionProcess returns.

Current behaviour needs to be checked (I suppose there is no committing of buffered cursors) and most likely such a fallback commit mechanism needs to be implemented.