Fixing output from cobra, when error occured.

This fixes following problem:
```
$ sti usage --unknown
unknown flag: --unknown

Usage: 
  sti [flags]
  sti [command]

Available Commands: 
  version                      Display version
  build <source> <image> <tag> Build a new image
  usage <image>                Print usage of the assemble script associated with the image
  help [command]               Help about any command

 Available Flags:
  -h, --help=false: help for sti
      --loglevel=0: Set the level of log output (0-3)
      --savetempdir=false: Save the temporary directory used by STI instead of deleting it
  -U, --url="unix:///var/run/docker.sock": Set the url of the docker socket to use

Use "sti help [command]" for more information about that command.
F1209 14:18:06.791831 23686 main.go:146] An error occurred: unknown flag: --unknown
Usage of sti:
      --loglevel=0: Set the level of log output (0-3)
      --savetempdir=false: Save the temporary directory used by STI instead of deleting it
  -U, --url="unix:///var/run/docker.sock": Set the url of the docker socket to use
```

The bottom line (starting with `F1209 14:18:06.79...`) is unnecessary, since cobra already provides all the information for us.