api: panic instead of returning error

From a usability perspective, it is simpler to do direct assignment than to assign / return `if err != nil`. Currently Columnize has one case where it might return error, and that is when an unexpected type is passed to it. Expected behavior in this case would be to panic, since the interface has been misused.

This means this change will be backward-incompatible, since it will no longer be necessary to return errors.