Throw an exception if Parser::disableCache is called before mOutput initialized ...

Throw an exception if Parser::disableCache is called before mOutput initialized (Aka when not parsing something).

So far I've encountered 2 extensions that give fatal errors from calling $wgParser->disableOutput() from hooks that are called at points where parsing is not taking place! Exception with a backtrace is much nicer than "Fatal error: Call to a member function disableCache() on a non-object..."