Unhelpful error on ambiguous type-parametrised capability

The following program reports an error that "get[T] is not allowed in this context",
even though there are `get[A]` _and_ `get[B]` available in the context.
The real issue is, I assume, that there's no _unique_ capability in sight.

Related to #1194.

```scala
effect get[T](): Int
def foo[A, B](): Int / {get[A], get[B]} =
do get() // <- error: Effect get[T] is not allowed in this context.
```

and same error here:
```scala
effect get[T](): Int
effect got { body: [A, B] () => Int / {get[A], get[B]} }: Int
def foo[A, B]: Int / got = do got { [X, Y] () => do get() } // <- error: Effect get[T] is not allowed in this context.
```

and here:
```scala
effect get[T](): Int
def foo[A, B](): Int = try {
do get() // <- error: Effect get[T] is not allowed in this context.
} with get[A] { resume(42)
} with get[B] { resume(100) }
```