java.lang.ClassCastException when handling an effect alias of an interface

Effekt crashes with the following error if you incorrectly try to use an effect that's aliased to an interface:

`Exception in thread "main" java.lang.ClassCastException: class effekt.symbols.EffectAlias cannot be cast to class effekt.symbols.BlockTypeConstructor$Interface (effekt.symbols.EffectAlias and effekt.symbols.BlockTypeConstructor$Interface are in unnamed module of loader 'app')`

On the web, it crashes with `TypeError: can't access property "a", a is undefined`.

[Reproduction](https://effekt-lang.org/playground.html?playground=TY5BCsIwEEX3c4q%2FTFeCuCr0AG5FDxCTiQbNVOIUEendTaLQ7obhvZkXRTkH6xhHzgkfAjwHyJSM9NiLdj1OEpVmIg6BneLA1p%2Btu2FoDlEVLqP5k9isCT%2B2W7tt9%2BOSjWI6DOWP5nfTyviKel2sGrHKKDQyP6fE5pFL713KrlrzFw%3D%3D&repl=bWFpbigp):

```scala
interface Term {
def num(n: Int): Unit
}

effect Readback = Term

def main() =
try go()
with Readback {
def num(n) = resume(println(n))
}
```