(bug 47449) {{#special:}} should always return a usable title

{{#special:}} should always return a usable title in NS_SPECIAL,
even when given an unknown special page title. Currently, it returns
"No such special page" text for such unknown names, which is
surprising.

Bug: 47449
Change-Id: I7f184956ec9d56c849f201f1230736fcedd0fadf