DownloadingSucceededEventListener: improve error handling

[php-coder]

We should resolve the following TODOs:

mystamps/src/main/java/ru/mystamps/web/controller/event/DownloadingSucceededEventListener.java

Lines 61 to 66 in 29f6dc9

	String content = importService.getDownloadedContent(requestId);
	if (content == null) {
	// TODO: how to handle error? maybe publish UnexpectedErrorEvent?
	log.error("Request #{}: could not load a content from database", requestId);
	return;
	}

and

mystamps/src/main/java/ru/mystamps/web/controller/event/DownloadingSucceededEventListener.java

Lines 77 to 81 in 29f6dc9

	if (parser == null) {
	// TODO: how to handle error? maybe publish UnexpectedErrorEvent?
	log.error("Request #{}: could not find appropriate parser", requestId);
	return;
	}

Tech debt for 29f6dc9 (#660)