[fix] Fix a crash when command directory isn't found

Trying to run an empty command with `flatiron.plugins.cli` when the
commands directory wasn't there resulted in the following crash:

    $ bin/bender-cli

    /Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:201
                  'Cannot find commands for ' + name.magenta
                                                    ^
    TypeError: Cannot read property 'magenta' of undefined
        at loadCommand (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:201:49)
        at executeCommand (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:278:26)
        at Object.app.router.notfound (/Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:357:5)
        at Router.dispatch (/Users/maciej/dev/js/flatiron/node_modules/director/lib/director/cli.js:50:21)
        at /Users/maciej/dev/js/flatiron/lib/flatiron/plugins/cli.js:87:18
        at onComplete (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/app.js:85:5)
        at Object.exports.ensure (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/features/index.js:10:10)
        at ensureFeatures (/Users/maciej/dev/js/flatiron/node_modules/broadway/lib/broadway/app.js:91:18)
        at /Users/maciej/dev/js/flatiron/node_modules/broadway/node_modules/utile/node_modules/async/lib/async.js:116:25
        at /Users/maciej/dev/js/flatiron/node_modules/broadway/node_modules/utile/node_modules/async/lib/async.js:24:16