print valid names

As a help for users, so that they do not get 

    Unknown release: 'fuerte'

and start trying numeric values, or something. 

I also though about changing the "Unknown release:" prefix for known but unsupported distros like fuerte, electric, diamondback, etc., but I am not passionate about this, and just changed roslocate to consider the prefix as it is. If that wrre to be improved, I'd vote for specific Error classes rather than RuntimeError.