Confusing error message for abstract method without return type

The following is legal in `scalac` and creates a method with return type `Unit`:

``` scala
trait Message {
  def first(x: Int)
}
```

In Dotty, it's illegal, I agree with that, but the error message should tell you that you're missing a return type, instead you get:

``` scala
confusing.scala:3: error: '=' expected but '}' found.
 }
 ^
one error found
```

which is hard to diagnose.
