login: clarify error message when user does not exist
login: clarify error message when user does not exist

Change-Id: I248d11a7ac13b9a14c45d2f8d481a65adb5f3fb9

PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,magul/pywikibot-core,happy5214/pywikibot-core,happy5214/pywikibot-core,npdoty/pywikibot,darthbhyrava/pywikibot-local,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,jayvdb/pywikibot-core,wikimedia/pywikibot-core,Darkdadaah/pywikibot-core,magul/pywikibot-core,smalyshev/pywikibot-core,h4ck3rm1k3/pywikibot-core,npdoty/pywikibot