* 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
I needed this to be able to self-validate without posting (no url.Values, I received the data from a JSON posted in body), but I wanted to use the same validation routine.
I called it RawValue, because that's the name used in Validator, but I can change it.