Display page errors in the web console

Fixes issues #55 and #100 and also corrects the timestamps for console API messages.

One potentially confusing bit is that network-related errors appear to belong in the JS category, but this is a side-effect of the very specific type of network events we display for Gecko. These are not regular logs of network transfers, but plain error messages when things break. I don't believe we have any similar messages in Gecko, but if we do, I bet they end up in the JS category as well.

When I get to add support for network traffic, I may revisit this, but it seems useful to display errors separately from traffic data, as the latter can be overwhelming and therefore disabled.