Closes #1684

This adds code to make sure the user is notified if he or she does not have chrome or firefox and needs to download one of them to run Lantern.

This goes a bit beyond the original ticket in that it adds a check for firefox and open Lantern in FireFox if it finds it. 

If you can have a look @oxtoacart that would be great!
This is the condition I'm talking about.  Today, when executable == null, we fall into the branch where we call openSystemDefaultBrowser().