* Change RenderField to use variadic parameter, so it can be called from...

... a template using only one parameter

* Allows manual field error setting to BaseForm (for exemple, to show API errors)
* Fields retain the entered value even on error
`Raw` sounds like this is original unvalidated (and possibly unfiltered) value, which is not the case here. I think it is better to call this method `InterfaceValue` or `IfaceValue`.