Better errors for missing/wrong JSON properties

It would be nice if we could better understand what's wrong with an incoming JSON body and throw a reasonable error about it. For now, we throw NotParsed(BodyItem, ....). Would be nice if we could promote BodyItem to a case class BodyItem(name: String) and wrap missing/wrongly decoded properties in it.

This is only about the cases when JSON is decoded directly to a case class: body.as[User]. I believe, end users should handle this manually (with custom exceptions like NotFoundJsonProperty) when dealing with raw JSON types directly: body.as[Json] and then parsing them with AST API provided by a JSON library.

See discussion in #261 started by @lderks.