log.js: fix inconsistency on page refresh

 - Change the filter to "errors only"
 - Hit refresh
 -> Filter will remain on "errors only"; the actual filtering should
    reflect that.

We forgot to check the "initial" state of the select box after installing
the event handler.  Fix that.

I'm not bothering to fix the flash-of-unbehavioured-content right now.
I think that's a fairly obvious temporary glitch compared to the
head-scratching inconsistency here.