Correct error handling incorrect environment for "github pin" command

> knife github pin xxx_motd 1.1.0 XXX
> Pin version 1.1.0 of cookbook xxx_motd in Environment XXX? (Y/N) y
> ERROR: knife encountered an unexpected error
> This may be a bug in the 'github pin' knife command or plugin
> Please collect the output of this command with the `-VV` option before filing a bug report.
> Exception: NoMethodError: undefined method `cookbook_versions' for nil:NilClass
