Shorter and clearer error types.
- InvalidParameterError is now RefError. It is returned when a ref (the
new ref or the parent ref) is invalid.
- The ResolveRefError is not renamed. It is returned when a ref can't
be resolved (e.g., ref not found at URL).
New error type ParameterComboError. It is returned when a parent ref is
specified but no URL, which is an invalid parameter combination.
