generate_user_config.py: Check a previous user-password.py
generate_user_config.py: Check a previous user-password.py

If file exists, it'll simply print a warning (using file_exists() fct)
and bypass the filling of botpasswords.

Bug: T167573
Change-Id: I71606c0075e49ac1a7161c9bcbeace3ae2d3dc45

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