No supertype allowed in handler return

The following program gives the annotated error:
```effekt
interface Done {
  def done(): Nothing
}

def main() = {
  try {
    do done()
  } with Done {
    def done() = ()   // Error: Expected Nothing but got Unit.
  }
}
```

Is this the behaviour we want?

Note that this also disallows, e.g., a hole instead of `do done()`.