Really log calls to wfDebugLog()

https://wikia-inc.atlassian.net/browse/PLATFORM-424

@Wikia/platform-team 

Start logging calls to ``wfDebugLog()``
Should "onDebug" log something at the "error" level? 

It also seems a little strange to me to throw an Exception only to catch it for the backtrace it generates. Can we use debug_backtrace(), or do something like 

```php
$exception = new \Exception();
WikiaLogger::instance()->debug(trim($text), ['exception' => $exception]);
```