VE-fake-pageview: Better reporting for fake pageviews

https://wikia-inc.atlassian.net/browse/VE-907
Just a suggestion: this is not directly VE code, so VE JS styling conventions do not apply - but: do we have a convention of prepending optional arguments with "opt_"? If not then it might be better to follow any convention instead of "creating" new one. In case of VE it would be to name parameter as every other (required) parameter, and imply that it is optional purely in comment, by surrounding it's name with brackets. For example look at method: ve.init.mw.Target.prototype.tryWithPreparedCacheKey