For returning from a Web API 2 controller, I can return content with the response if the response is OK (status 200) like this:
public IHttpActionResult
Sorry for the late answer why don't you simple use
return BadRequest("your message");
I use it for all my IHttpActionResult errors its working well
IHttpActionResult
here is the documentation : https://msdn.microsoft.com/en-us/library/system.web.http.apicontroller.badrequest(v=vs.118).aspx