A key for request attribute that contains any exception
that might have occured before the handling has been
propagated to ScalatraBase#handle (such as in
FileUploadSupport)
valRenderCallbacks: String
defaddCallback(callback: (Try[Any]) ⇒ Unit)(implicit request: HttpServletRequest): Unit
defaddRenderCallback(callback: (Try[Any]) ⇒ Unit)(implicit request: HttpServletRequest): Unit