[IMPR] use site.watch() when calling site.watchpage()
[IMPR] use site.watch() when calling site.watchpage()

- use site.watch() when calling site.watchpage() and remove code
  duplication.
- self.watchpage is old enough to ommit the detailed api response.
  site.watch should be used in that case which raises a KeyError
  in such case.

Change-Id: I1143f0a8fa5c409760a656f154f28b79ad2ece94

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