Don't print help for unknown commands

In case you try executing unknown command (for example appbuilder unknown) we show error message that such command does not exist and after that we are trying to find the help for this unknown command, which is not correct.

Related to 9d17203744077edcf7f034ec58af4a2d6f0a64a7

Fixes http://teampulse.telerik.com/view#item/281820