Print a friendly error when running whoami without authorizing first

New users who run `t whoami` without running `t authorize` first
currently see a nasty error:

    spectre256@laguna ~ $ t whoami
    /home/spectre256/.gem/ruby/2.1.0/gems/t-2.8.0/lib/t/cli.rb:928:in `whoami': undefined method `[]' for nil:NilClass (NoMethodError)
    	from /home/spectre256/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
    	from /home/spectre256/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
    	from /home/spectre256/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
    	from /home/spectre256/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
    	from /home/spectre256/.gem/ruby/2.1.0/gems/t-2.8.0/bin/t:20:in `<top (required)>'
    	from /home/spectre256/.gem/ruby/2.1.0/bin/t:23:in `load'
    	from /home/spectre256/.gem/ruby/2.1.0/bin/t:23:in `<main>'

Now instead they'll see a friendly, helpful message

    spectre256@laguna ~/repos/t $ bundle exec t whoami
    You haven't authorized an account, run `t authorize` to get started.


I'm happy to make any tweaks if there are preferences for setting up the test, checking for valid configuration, or printing to stderr. Thanks!
`""` makes RuboCop fail. You should use `''` instead.