Fix browser problem

This is a partial fix for Issue #128 - Don't work in ubuntu 13.04.

The problem is that new Browser will throw an exception when it can't find a proper browser. This will prevent that exception from interfering with the rest of the preferences configuration (although the preview pane is still missing).

I'll try to add a better message when it can't load the browser, explaining what options the user has, but in the meantime this fixes the case where you have to load the preferences pane twice.