harvest_template.py: template error handling
harvest_template.py: template error handling

- do not process pages if the template doesnt exist
- catch and warn when extract_templates_and_params cant detect
  template name correctly
- do not process page template invocation when field or value is empty

Change-Id: Id83cbe01e553655f3aedbb989dd675bcae2a7327

happy5214/pywikibot-core,Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,h4ck3rm1k3/pywikibot-core,darthbhyrava/pywikibot-local,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,jayvdb/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,emijrp/pywikibot-core,smalyshev/pywikibot-core,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,happy5214/pywikibot-core,hasteur/g13bot_tools_new,trishnaguha/pywikibot-core,xZise/pywikibot-core,VcamX/pywikibot-core,icyflame/batman,magul/pywikibot-core,npdoty/pywikibot,TridevGuha/pywikibot-core,magul/pywikibot-core,valhallasw/pywikibot-core