Return exit status of 1 on invalid command

Makes `vagrant destroyjj` and similar invalid commands return an exit code of 1, so that `vagrant destroyjj && vagrant up` works as expected.  (i.e. the `vagrant up` is not executed.)

I don't really know Ruby so the test could be complete rubbish.  Are there really no existing tests for `Vagrant::CLI`?  I couldn't find any.