Currently, commands such as `ks env non-existent-subcommand` will output
an error `Command 'env' requires a subcommand`.

This is not helpful since the user may assume there is the subcommand
'non-existent-subcommand'. This commit will clarify the error messages
in this scenario.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>