site_detect.py: Raise ServerError for 502 response status
site_detect.py: Raise ServerError for 502 response status

Bug: T175584
Change-Id: I1f2d5cb701f37c5e7e40db81b0849d9989485c2c

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