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.