Factories should produce better error messages for semi-compatible arguments

[pshirshov]

 trait Super
 trait Sub extends Super

 case class Output(s: Super)

  trait Factory {
    def make(arg: Sub): Output
  }

arg will not be captured by the macro and the error message isn't easy to read/interpret. We should improve the error message and, possibly, allow non-ambiguous subtypes in such cases.