Fix "undefined method `IOError'" on bad dir

If you tried to run on a directory that wasn't from a twitter archive you would get 

    lib/grailbird_updater.rb:23:in `read_required': undefined method `IOError' for GrailbirdUpdater::JsFile:Class (NoMethodError)

Now you get the expected

    lib/grailbird_updater.rb:23:in `read_required': ./data/js/user_details.js must exist (IOError)