Compiler panic "?C should be a concrete capture set in this phase."

In some larger code base I got the following panic:

```
[error] ?C should be a concrete capture set in this phase.
[info] at effekt.util.messages$CompilerPanic$.apply(Messages.scala:37)
[info] at effekt.util.messages$ErrorReporter.panic(Messages.scala:118)
[info] at effekt.util.messages$ErrorReporter.panic$(Messages.scala:99)
[info] at effekt.context.Context.panic(Context.scala:41)
[info] at effekt.util.messages$ErrorReporter.panic(Messages.scala:119)
[info] at effekt.core.Transformer$.transform(Transformer.scala:861)
[info] at effekt.core.Transformer$.transform(Transformer.scala:835)
[info] at effekt.core.Transformer$.coercing$$anonfun$2(Transformer.scala:911)
[info] at effekt.core.Transformer$.$anonfun$65(Transformer.scala:828)
[info] at effekt.core.TransformerOps.withBindings$$anonfun$1(Transformer.scala:986)
[info] at effekt.context.ContextOps.in(Context.scala:31)
[info] at effekt.context.ContextOps.in$(Context.scala:17)
[info] at effekt.context.Context.effekt$namer$NamerOps$$super$in(Context.scala:41)
[info] at effekt.namer.NamerOps.in(Namer.scala:979)
[info] at effekt.namer.NamerOps.in$(Namer.scala:961)
...
```

I minimized the responsible code to the following:

```scala
effect break(): => Int at {}

def main() = {
try do break()
with break { resume(42 match {
case 42 => box { 42 }
case _ => panic("whaa")
}) }
()
}
```

Not sure if this is even supposed to work, but we should catch this in earlier phases either way.