Front-end part of JS error logging.

~200 bytes are added to gzipped HTML response if the error collecting is enabled  via wg varliable. It binds to onerror event and sends 1% of errors encoded as JSON to the URL configured: http://jserrorslog.wikia.com/l?[errorMsg, fileUrl, lineNumber, varnishServer].

The change is experimental and will only be present on Oasis (we should extend to other skins later).