[FEAT] Show a warning if user does not exists on a given site
[FEAT] Show a warning if user does not exists on a given site

This is a more general solution of T191950

Bug: T191950
Change-Id: Ib2c74c580f22c393b4fee99d53260f2f71d07c06

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core