When URL of not existing Wikipedia/Commons page is inserted, tool display error message "error.page-missing".
Some real error message would be nice.
This is also the case for pages like https://commons.wikimedia.org/wiki/User:Christoph_Fischer_%28WMDE%29 which does not exist in Commons but when this URL is opened in the browser, Commons show user page from Meta. Possibility to handle such user pages could be also investigated.