Fixed implicit marshalling of `Either` results with proper status setting

Currently the `implicit def errorSelectingEitherMarshaller[A, B](implicit ...` conversion is never used in the code hence there's no proper setting of the request status code. Refer to the [spray documentation on `ToResponseMarshaller`](http://spray.io/documentation/1.2.1/spray-httpx/marshalling/#toresponsemarshaller)

Also the once the conversion started working the only code returned was '500 Internal Server Error', so the result code setting had to be corrected, too.