Handle invalid CLI flags (#421)
Handle invalid CLI flags (#421)

Previously, the flags were not handled so the application would
Looking at the documentation for https://github.com/urfave/cli , the error is propogated up through the `app.Run` call.
Now, the error is checked before fatally logging it.

closes https://github.com/aelsabbahy/goss/issues/420
aelsabbahy/goss,aelsabbahy/goss