Issue Reference: https://github.com/spf13/cobra/issues/1056

https://github.com/spf13/cobra/pull/922 introduced a new error
type that emitted when a command was not runnable. This caused
all commands w/o a run function set to error w/ that message and a status code of 1.

This change reverts the addition of that new error. Similar
functionality can be accomplished by leveraging RunE.