page.py: Raise NoPage in WikibasePage.get if the entity does not exist
page.py: Raise NoPage in WikibasePage.get if the entity does not exist

The test "test_item_never_existed" in "wikibase_tests.py" was failing
because WikibasePage.get was raising APIError instead.

Change-Id: Ie36e0b6f9898ab7eb3c74ae81ba99ee7a5880ef6

wikimedia/pywikibot-core,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,npdoty/pywikibot,npdoty/pywikibot,magul/pywikibot-core,happy5214/pywikibot-core,jayvdb/pywikibot-core,magul/pywikibot-core,Darkdadaah/pywikibot-core,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,PersianWikipedia/pywikibot-core