It seems that querying /w/api.php?action=query&prop=pageviews&title=Does_not_exists still ends up hitting the RESTBase backends, and logging messages to the PageViewInfo channel with level ERROR, indicating a server failure, when this is in fact a non-error, and not worth logging to Logstash, given it is a client error (Page does not exist), which we don't normally log.
Sample for https://de.wikipedia.org/w/index.php?title=Johan_Landsberg&action=info
Failed fetching https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/de.wikipedia.org/all-access/user/Johan_Landsberg/daily/20170921/20171119: There was a problem during the HTTP request: 404 Not Found type mediawiki channel PageViewInfo level ERROR url https://de.wikipedia.org//w/index.php?title=Johan_Landsberg&action=info apierror_detail The date(s) you used are valid, but we either do not have data for those date(s), or the project you asked for is not loaded yet. Please check https://wikimedia.org/api/rest_v1/?doc for more information.
Sample for https://en.wikivoyage.org/w/api.php?action=query&titles=Talk:Example&prop=pageviews&format=json
Failed fetching https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/en.wikivoyage.org/all-access/user/Talk%3AExample/daily/20170920/20171118: There was a problem during the HTTP request: 404 Not Found