proofreadpage.py: purge IndexPage when Index has no required class set
proofreadpage.py: purge IndexPage when Index has no required class set

Purge index if class="qualityN prp-pagequality-N" is not found in HTML
parsing of IndexPage.

Raise ValueError if page.purge() has no effect.

Bug: T114318
Change-Id: I342df4e5b5e2743dbb5bdb8f55296e07242b6b58

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