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()`.