an illegal root section name crashes the software

[lelutin]

Hello, when trying out this software I made the simple mistake of spelling the root section name wrong. Instead of saying that the section name does not exist, the software just crashed like so:

vagrant@jessie:~/Project-Illuminati$ bundle exec ruby nwo.rb -f p2.yml -n koumbitswitches
/home/vagrant/Project-Illuminati/lib/illuminati/p2.rb:5:in `initialize': undefined method `[]' for nil:NilClass (NoMethodError)
	from nwo.rb:15:in `new'
	from nwo.rb:15:in `<main>'

it'd be nice to check that the root section name exists before calling initialize and if it doesn't exist tell something like this to the user: "error: root section name %s does not exist"