STORM-488: Exit with 254 error code if storm CLI is run with unknown command

This change follows the existing semantics when the `storm` CLI tool is invoked without _any_ parameters.

With this change, whenever the user tries to run `storm <cmd>` with an unknown command the `storm` script will print the usage help (existing behavior) AND then exit with a 254 error code.

I decided not to re-use the 255 error code, which is returned by the `storm` CLI tool when being invoked without any parameters.  Having two separate error codes allows downstream applications (including Ansible, Puppet) to better understand what went wrong and react accordingly -- e.g. giving the user more meaningful feedback.