_Util.DOM.getBBox() doesn't choke on Firefox.

Previously, calling getBBox() on a node with the style "display: none"
would throw an error in Firefox (FF bug 612118). The utility method
now returns a safe value in that case, bringing FF's behavior in line
with that of other browsers.

Addresses Slate issue 840.
Doesn't catch the error unless you catch `Object`, because the NS_ERROR is not `instanceof Error` :(

On top of that, if you do catch it, you get an unhelpful error message.